Prenatal Development

Hidden from view, the process of prenatal development has always been mysterious and fascinating, and beliefs about the origins of human life and development before birth have been an important part of the lore and traditions of all societies. (Box 2.1 describes one set of cultural beliefs about the beginning of life that is quite unlike those of Western societies.)

FIGURE 2.1 Preformationism A seventeenth-century drawing of a preformed being inside a sperm. This drawing was based on the claim of committed preformationists that when they looked at samples of semen under the newly invented microscope, they could actually see a tiny figure curled up inside the head of the sperm. They believed that the miniature person would enlarge after entering an egg. As this drawing illustrates, we must always take care not to let our cherished preconceptions so dominate our thinking that we see what we want to see—not what is really there. (From Moore & Persaud, 1993, p. 7)
MARY EVANS PICTURE LIBRARY/ALAMY

epigenesis the emergence of new structures and functions in the course of development

When we look back in history, we see great differences in how people have thought about prenatal development. In the fourth century b.c.e., Aristotle posed the fundamental question about prenatal development that was to underlie Western thought about it for the next 15 centuries: Does prenatal life start with the new individual already preformed, composed of a full set of tiny parts, or do the many parts of the human body develop in succession? Aristotle rejected the idea of preformation in favor of what he termed epigenesis—the emergence of new structures and functions during development (we will revisit this idea in Chapter 3 in its more modern form, epigenetics). Seeking support for his idea, he took what was then a very unorthodox step: he opened fertile chicken eggs to observe chick organs in various stages of development. Nevertheless, the idea of preformation persisted long after Aristotle, degenerating into a dispute about whether the miniature, preformed human was lodged inside the mother’s egg or the father’s sperm (see Figure 2.1).

The notion of preformation may strike you as simpleminded. Remember, however, that our ancient forebears had no way of knowing about the existence of cells and genes or about behavioral development in the womb. Many of the mysteries that perplexed our ancestors have now been solved, but as is always true in science, new mysteries have replaced them.

Box 2.1: a closer look: BENG BEGINNINGS

Few topics have generated more intense debate and dispute in the United States in recent years than the issue of when life begins—at the moment of conception, the moment of birth, or sometime in between. The irony is that few who engage in this debate recognize how complex the issue is or the degree to which societies throughout the world have different views on it.

The mother of this Beng baby has spent considerable time painting the baby’s face in an elaborate pattern. She does this every day in an effort to make the baby attractive so other people will help keep the baby happy in this world.
COURTESY OF ALMA GOTLIEB

Consider, for example, the perspective of the Beng, a people in the Ivory Coast of West Africa, who believe that every newborn is a reincarnation of an ancestor (Gottlieb, 2004). According to the Beng, in the first weeks after birth, the ancestor’s spirit, its wru, is not fully committed to an earthly life and therefore maintains a double existence, traveling back and forth between the everyday world and wrugbe, or “spirit village.” (The term can be roughly translated as “afterlife,” but “before-life” might be just as appropriate.) It is only after the umbilical stump has dropped off that the newborn is considered to have emerged from wrugbe and to be a person. If the newborn dies before this point, there is no funeral, for the infant’s passing is perceived as a return to the wrugbe.

These beliefs underlie many aspects of Beng infant-care practices. One is the frequent application of an herbal mixture to the newborn’s umbilical stump to hasten its drying out and dropping off. In addition, there is the constant danger that the infant will become homesick for its life in wrugbe and decide to leave its earthly existence. To prevent this, parents try to make their babies comfortable and happy so they will want to stay in this life. Among the many recommended procedures is elaborately decorating the infant’s face and body to elicit positive attention from others. Sometimes diviners are consulted, especially if the baby seems to be unhappy; a common diagnosis for prolonged crying is that the baby wants a different name—the one from its previous life in wrugbe.

So when does life begin for the Beng? In one sense, a Beng individual’s life begins well before birth, since he or she is a reincarnation of an ancestor. In another sense, however, life begins sometime after birth, when the individual is considered to have become a person.

Video: Beng Infant Caretaking Practices: Carrying Babies

Video: Beng Infant Caretaking Practices: Playing

Video: Beng Infant Caretaking Practices: Speech

Video: Beng Infant Caretaking Practices: Jewelry

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Conception

gametes (germ cells) reproductive cells—egg and sperm—that contain only half the genetic material of all the other cells in the body

meiosis cell division that produces gametes

Each of us originated as a single cell that resulted from the union of two highly specialized cells—a sperm from our father and an egg from our mother. These gametes, or germ cells, are unique not only in their function but also in the fact that each one contains only half the genetic material found in other cells. Gametes are produced through meiosis, a special type of cell division in which the eggs and sperm receive only one member from each of the 23 chromosome pairs contained in all other cells of the body. This reduction to 23 chromosomes in each gamete is necessary for reproduction, because the union of egg and sperm must contain the normal amount of genetic material (23 pairs of chromosomes). A major difference in the formation of these two types of gametes is the fact that almost all the eggs a woman will ever have are formed during her own prenatal development, whereas men produce vast numbers of new sperm continuously.

conception the union of an egg from the mother and a sperm from the father

The process of reproduction starts with the launching of an egg (the largest cell in the human body) from one of the woman’s ovaries into the adjoining fallopian tube (see Figure 2.2). As the egg moves through the tube toward the uterus, it emits a chemical substance that acts as a sort of beacon, a “come-hither” signal that attracts sperm toward it. If an act of sexual intercourse takes place near the time the egg is released, conception, the union of sperm and egg, will be possible. In every ejaculation, as many as 500 million sperm are pumped into the woman’s vagina. Each sperm, a streamlined vehicle for delivering the man’s genes to the woman’s egg, consists of little more than a pointed head packed full of genetic material (the 23 chromosomes) and a long tail that whips around to propel the sperm through the woman’s reproductive system.

FIGURE 2.2 Female reproductive system A simplified illustration of the female reproductive system, with a fetus developing in the uterus (womb). The umbilical cord runs from the fetus to the placenta, which is burrowed deeply into the wall of the uterus. The fetus is floating in amniotic fluid inside the amniotic sac.

To be a candidate for initiating conception, a sperm must travel for about 6 hours, journeying 6 to 7 inches from the vagina up through the uterus to the egg-bearing fallopian tube. The rate of attrition on this journey is enormous: of the millions of sperm that enter the vagina, only about 200 ever get near the egg (see Figure 2.3). There are many causes for this high failure rate. Some failures are due to chance: many of the sperm get tangled up with other sperm milling about in the vagina; others wind up in the fallopian tube that does not currently harbor an egg. Other failures have to do with the fact that a substantial portion of the sperm have serious genetic or other defects that prevent them from propelling themselves vigorously enough to reach and fertilize the egg. Thus, any sperm that do get to the egg are relatively likely to be healthy and structurally sound, revealing a Darwinian-type “survival of the fittest” process operating during fertilization. (Box 2.2 describes the consequences of this selection process for the conception of males and females.)

FIGURE 2.3 (a) Sperm nearing the egg Of the millions of sperm that started out together, only a few ever get near the egg. The egg is the largest human cell (the only one visible to the naked eye), but sperm are among the smallest. (b) Sperm penetrating the egg This sperm is whipping its tail around furiously to drill itself through the outer covering of the egg.
LENART NILSON/SCANPIX
LENART NILSON/SCANPIX

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Box 2.2: individual differences: THE FIRST—AND LAST—SEX DIFFERENCES

The proverbial competition between the sexes might be said to begin with millions of sperm racing to fertilize the egg. Sperm that carry a Y chromosome (the genetic basis for maleness) are lighter and swim faster than those bearing an X chromosome, so the race to the egg is won much more often by the “boys.” As a result, approximately 120 to 150 males are conceived for every 100 females.

Beginning at birth, the U.S. male-to-female mortality ratio exceeds 1 across the life span. The spike that occurs in adolescence and early adulthood—peaking at 3 male deaths for every female death—is largely the result of external causes, such as accidents, homicide, and suicide.

The girls win the next big competition—survival. The ratio at birth is only 106 males to 100 females. Where are the missing males? Obviously, they are miscarried at a much greater rate than females. Birth is also more challenging for boys, who, usually because of possible birth complications, are 50% more likely to need to be surgically removed from the womb by means of a cesarean delivery. This heightened vulnerability is not limited to surviving the prenatal period. Boys also suffer disproportionately from most developmental disorders, including language and learning disorders, dyslexia, attention-deficit disorder, intellectual disabilities, and autism. The greater fragility of males continues throughout life, as reflected in the graph. Adolescent boys are more impulsive and take more risks than girls, and they are more likely to commit suicide or die violently.

Differential survival is not always left in the hands of nature. In many societies, both historically and currently, male offspring are more highly valued than females, and parents resort to infanticide to avoid having daughters. For example, Inuit families in Alaska traditionally depended on male children to help in the hunt for food, and in former times, Inuit girls were often killed at birth. Over the past several decades, the Chinese government strictly enforced a “one-child” policy, a measure designed to reduce population growth by forbidding couples to have more than one child. This policy resulted in many parents killing or abandoning their female babies (or giving them up for adoption to Western families) in order to make room for a male child. A more technological approach is currently practiced in some countries that place a premium on male offspring: prenatal tests are used to determine the gender of the fetus, and female fetuses are selectively aborted. These cases dramatically illustrate the sociocultural model of development described in Chapter 1, showing how cultural values, government policy, and available technology all affect developmental outcomes.

zygote a fertilized egg cell

As soon as one sperm’s head penetrates the outer membrane of the egg, a chemical reaction seals the membrane, preventing other sperm from entering. The tail of the sperm falls off, the contents of its head gush into the egg, and the nuclei of the two cells merge within hours. The fertilized egg, known as a zygote, now has a full complement of human genetic material, half from the mother and half from the father. The first of the three periods of prenatal development (see Table 2.1) has begun and, if everything proceeds normally, that development will continue for approximately 9 months (on average, 38 weeks or 266 days).

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Developmental Processes

embryo the name given to the developing organism from the 3rd to 8th week of prenatal development

fetus the name given to the developing organism from the 9th week to birth

Before describing the course of prenatal development, we need to briefly outline four major developmental processes that underlie the transformation of a zygote into an embryo and then a fetus. The first is cell division, known as mitosis. Within 12 hours or so after fertilization, the zygote divides into two equal parts, each containing a full complement of genetic material. These two cells then divide into four, those four into eight, those eight into sixteen, and so on. Through continued cell division over the course of 38 weeks, the barely visible zygote becomes a newborn consisting of trillions of cells.

A second major process, which occurs during the embryonic period, is cell migration, the movement of newly formed cells away from their point of origin. Among the many cells that migrate are the neurons that originate deep inside the embryonic brain and then, like pioneers settling new territory, travel to the outer reaches of the developing brain.

mitosis cell division that results in two identical daughter cells

The third process in prenatal development is cell differentiation. Initially, all of the embryo’s cells, referred to as embryonic stem cells, are equivalent and interchangeable: none has any fixed fate or function. After several cell divisions, however, these cells start to specialize in terms of both structure and function. In humans, embryonic stem cells develop into roughly 350 different types of cells, which perform particular functions on behalf of the organism. (Because of their developmental flexibility, embryonic stem cells are currently the focus of a great deal of research in regenerative medicine. The hope is that when injected into a person suffering from illness or injury, embryonic stem cells will develop into healthy cells to replace the diseased or damaged ones.)

embryonic stem cells embryonic cells, which can develop into any type of body cell

The process of differentiation is one of the major mysteries of prenatal development. Since all cells in the body have the identical set of genes, what factors determine which type of cell a given stem cell will become? One key determinant is which genes in the cell are “switched on” or expressed (see Box 2.3). Another is the cell’s location, because its future development is influenced by what is going on in neighboring cells.

The initial flexibility and subsequent inflexibility of cells, as well as the importance of location, is vividly illustrated by classic research with frog embryos. If the region of a frog embryo that would normally become an eye is grafted onto its belly area early in fetal development, the transplanted region will develop as a normal part of the belly. Thus, although the cells were initially in the right place to become an eye, they had not yet become specialized. If the transplant is performed later in fetal development, the same operation results in an eye—alone and unseeing—lodged in the frog’s belly (Wolpert, 1991).

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Box 2.3: a closer look: PHYLOGENETIC CONTINUITY

phylogenetic continuity the idea that because of our common evolutionary history, humans share many characteristics, behaviors, and developmental processes with other animals, especially mammals

Throughout this book, we will use research with nonhuman animals to make points about human development. In doing so, we subscribe to the principle of phylogenetic continuity—the idea that because of our common evolutionary history, humans share many characteristics and developmental processes with other living things. Indeed, you share most of your genes with your dog, cat, or hamster.

The assumption that animal models of behavior and development can be useful and informative for human development underlies a great deal of research. For example, much of our knowledge about the dangers of alcohol consumption by pregnant women comes from research with nonhuman animals. Because scientists suspected that drinking alcohol while pregnant caused the constellation of defects now known as fetal alcohol spectrum disorder, they experimentally exposed fetal mice to alcohol. At birth, these mice had atypical facial features, remarkably similar to the facial anomalies of human children heavily exposed to alcohol in the womb by their mother. This fact increased researchers’ confidence that the problems commonly associated with fetal alcohol syndrome are, in fact, caused by alcohol rather than by some other factor.

Scientists interested in human development have learned a great deal by studying maternal behavior in rats.
WILDLIFE GMBH/ALAMY

One of the most fascinating discoveries in recent years, discussed later in this chapter, is the existence of fetal learning. This phenomenon was first documented in one of comparative psychologists’ favorite creatures—the rat. To survive after birth, newborns must find a milk-producing maternal nipple. How do they know where to go? The answer is that they search for something familiar to them. During the birth process, the nipples on the underside of the mother rat’s belly get smeared with amniotic fluid. The scent of the amniotic fluid is familiar to the rat pups from their time in the womb, and it lures the babies to where they need to be—with their noses, and hence their mouths, near a nipple (Blass, 1990).

How was it determined that newborn rats find their mother’s nipple by recognizing the scent of amniotic fluid? For one thing, when researchers washed the mother’s belly clean of amniotic fluid, her pups failed to find her nipples, and if half her nipples were washed, the pups were attracted to the unwashed ones with amniotic fluid still on them (Blass & Teicher, 1980). Even more impressive, when researchers introduced odors or flavors into the amniotic fluid, either by directly injecting them or by adding them to the mother’s diet, her pups preferred those odors and tastes after birth (Hepper, 1988; Pedersen & Blass, 1982; Smotherman & Robinson, 1987). These and other demonstrations of fetal learning in rodents inspired developmental psychologists to look for similar processes in human fetuses. As you will see later, they found them.

apoptosis genetically programmed cell death

The fourth developmental process is something you would not normally think of as developmental at all—death. However, the selective death of certain cells is the “almost constant companion” to the other developmental processes we have described (Wolpert, 1991). The role of this genetically programmed “cell suicide,” known as apoptosis, is readily apparent in hand development (see Figure 2.4): the formation of fingers depends on the death of the cells in between the ridges in the hand plate. In other words, death is preprogrammed for the cells that disappear from the hand plates.

FIGURE 2.4 Embryonic hand plate Fingers will emerge from the hand plate of this 7-week-old embryo. The fingers are formed as a result of the death of the cells between the ridges you can see in the plate. If these cells did not expire, the baby would be born with webbed rather than independent fingers.
LENART NILSON/SCANPIX

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In addition to these four developmental processes, we need to call attention to the influence of hormones on prenatal development. For example, hormones play a crucial role in sexual differentiation. All human fetuses, regardless of the genes they carry, can develop either male or female genitalia. The presence or absence of androgens, a class of hormones that includes testosterone, causes development to proceed one way or the other. If androgens are present, male sex organs develop; if they are absent, female genitalia develop. The source of androgens is the male fetus itself. Around the 8th week after conception, the testes begin to produce these hormones, changing the developing organism forever. This is just one of the many ways in which the fetus influences its own development.

We now turn our attention to the general course of prenatal development that results from all the preceding influences, as well as other developmental processes.

Early Development

identical twins twins that result from the splitting in half of the zygote, resulting in each of the two resulting zygotes having exactly the same set of genes

On its journey through the fallopian tube to the womb, the zygote doubles its number of cells roughly twice a day. By the 4th day after conception, the cells arrange themselves into a hollow sphere with a bulge of cells, called the inner cell mass, on one side.

This is the stage at which identical twins most often originate. They result from a splitting in half of the inner cell mass, and thus they both have exactly the same genetic makeup. In contrast, fraternal twins result when two eggs happen to be released from the ovary into the fallopian tube and both are fertilized. Because they originate from two different eggs and two different sperm, fraternal twins are no more alike genetically than nontwin siblings with the same parents.

fraternal twins twins that result when two eggs happen to be released into the fallopian tube at the same time and are fertilized by two different sperm; fraternal twins have only half their genes in common.

neural tube a groove formed in the top layer of differentiated cells in the embryo that eventually becomes the brain and spinal cord

By the end of the 1st week following fertilization, if all goes well (which it does for less than half the zygotes that are conceived), a momentous event occurs—implantation, in which the zygote embeds itself in the uterine lining and becomes dependent on the mother for sustenance. Well before the end of the 2nd week, it will be completely embedded within the uterine wall.

After implantation, the embedded ball of cells starts to differentiate. The inner cell mass becomes the embryo, and the rest of the cells become an elaborate support system—including the amniotic sac and placenta—that enables the embryo to develop. The inner cell mass is initially a single layer thick, but during the 2nd week, it folds itself into three layers, each with a different developmental destiny. The top layer becomes the nervous system, the nails, teeth, inner ear, lens of the eyes, and the outer surface of the skin. The middle layer eventually becomes muscles, bones, the circulatory system, the inner layers of the skin, and other internal organs. The bottom layer develops into the digestive system, lungs, urinary tract, and glands. A few days after the embryo has differentiated into these three layers, a U-shaped groove forms down the center of the top layer. The folds at the top of the groove move together and fuse, creating the neural tube (Figure 2.5). One end of the neural tube will swell and develop into the brain, and the rest will become the spinal cord.

FIGURE 2.5 Neural tube In the 4th week, the neural tube begins to develop into the brain and spinal cord. In this photo, the neural groove, which fuses together first at the center and then outward in both directions as if two zippers were being closed, has been “zipped shut” except for one part still open at the top. Spina bifida, a congenital disorder in which the skin over the spinal cord is not fully closed, can originate at this point. After closing, the top of the neural tube will develop into the brain.
LENART NILSON/SCANPIX

amniotic sac a transparent, fluid-filled membrane that surrounds and protects the fetus

The support system that is emerging along with the embryo is elaborate and essential to the embryo’s development. One key element of this support system is the amniotic sac, a membrane filled with a clear, watery fluid in which the fetus floats. The amniotic fluid operates as a protective buffer for the developing fetus, providing it with a relatively even temperature and cushioning it against jolting. As you will see shortly, because the amniotic fluid keeps the fetus afloat, the fetus can exercise its tiny, weak muscles relatively unhampered by the effects of gravity.

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placenta a support organ for the fetus; it keeps the circulatory systems of the fetus and mother separate, but as a semipermeable membrane permits the exchange of some materials between them (oxygen and nutrients from mother to fetus and carbon dioxide and waste products from fetus to mother)

The second key element of the support system, the placenta, is a unique organ that permits the exchange of materials carried in the bloodstreams of the fetus and its mother. It is an extraordinarily rich network of blood vessels, including minute ones extending into the tissues of the mother’s uterus, with a total surface area of about 10 square yards—approximately the amount of driveway covered by the family car (Vaughan, 1996). Blood vessels running from the placenta to the embryo and back again are contained in the umbilical cord.

umbilical cord a tube containing the blood vessels connecting the fetus and placenta

At the placenta, the blood systems of the mother and fetus come extremely close to each other, but the placenta prevents their blood from actually mixing. However, the placental membrane is semipermeable, meaning that some elements can pass through it but others cannot. Oxygen, nutrients, minerals, and some antibodies—all of which are just as vital to the fetus as they are to you—are transported to the placenta by the mother’s circulating blood. They then cross the placenta and enter the fetal blood system. Waste products (e.g., carbon dioxide, urea) from the fetus cross the placenta in the opposite direction and are removed from the mother’s bloodstream by her normal excretory processes.

The placental membrane also serves as a defensive barrier against a host of dangerous toxins and infectious agents that can inhabit the mother’s body and could be harmful or even fatal to the fetus. Unfortunately, being semipermeable, the placenta is not a perfect barrier, and, as you will see later, a variety of harmful elements can cross it and attack the fetus. One other function of the placenta is the production of hormones, including estrogen, which increases the flow of maternal blood to the uterus, and progesterone, which suppresses uterine contractions that could lead to premature birth (Nathanielsz, 1994).

An Illustrated Summary of Prenatal Development

cephalocaudal development the pattern of growth in which areas near the head develop earlier than areas farther from the head

The course of prenatal development from the 4th week on is illustrated in Figure 2.6 through Figure 2.13, and significant milestones are highlighted in the accompanying text. (The fetal behaviors that are mentioned will be discussed in detail in the following section.) Notice that earlier development takes place at a more rapid pace than later development, and that the areas nearer the head develop earlier than those farther away (e.g., head before body, hands before feet)—a general tendency known as cephalocaudal development.

FIGURE 2.6 Embryo at 4 weeks
LENART NILSON/SCANPIX

Figure 2.6: At 4 weeks after conception, the embryo is curved so tightly that the head and the tail-like structure at the other end are almost touching. Several facial features have their origin in the set of four folds in the front of the embryo’s head; the face gradually emerges as a result of these tissues moving and stretching, as parts of them fuse and others separate. The round area near the top of the head is where the eye will form, and the round gray area near the back of the “neck” is the primordial inner ear. A primitive heart is visible; it is already beating and circulating blood. An arm bud can be seen in the side of the embryo; a leg bud is also present but less distinct.

FIGURE 2.7 Face development from 5½ to 8½ weeks
LENART NILSON FROM A CHILD IS BORN/BONIER FAKTA
LENART NILSON FROM A CHILD IS BORN/BONIER FAKTA

Figure 2.7: (a) In this 5½-week-old fetus, the nose, mouth, and palate are beginning to differentiate into separate structures. (b) Just 3 weeks later, the nose and mouth are almost fully formed. Cleft palate, one of the most common birth defects worldwide, involves malformations (sometimes minor, sometimes major) of this area. This condition originates sometime between 5½ and 8 weeks prenatally—precisely when these structures are developing.

FIGURE 2.9 Fetus at 11 weeks
ANATOMICAL TRAVELOGUE/PHOTO RESEARCHERS, INC.

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Figure 2.8: The head of this 9-week-old fetus overwhelms the rest of its body. The bulging forehead reflects the extremely rapid brain growth that has been going on for weeks. Rudimentary eyes and ears are forming. All the internal organs are present, although most must undergo further development. Sexual differentiation has started. Ribs are visible, fingers and toes have emerged, and nails are growing. You can see the umbilical cord connecting the fetus to the placenta. The fetus makes spontaneous movements, but because it is so small and is floating in amniotic fluid, the mother cannot feel them.

FIGURE 2.8 Fetus at 9 weeks
LENART NILSON/SCANPIX

Figure 2.9: This image of an 11-week-old fetus clearly shows the heart, which has achieved its basic adult structure. You can also see the developing spine and ribs, as well as the major divisions of the brain.

FIGURE 2.9 Fetus at 16 weeks
LENART NILSON/SCANPIX

Figure 2.10: During the last 5 months of prenatal development, the growth of the lower part of the body accelerates. The fetus’s movements have increased dramatically: its chest makes breathing movements, and some reflexes—grasping, swallowing, sucking—are present. By 16 weeks, the fetus is capable of intense kicks, although the mother feels them only as a mild “flutter.” At this age, the external genitalia are substantially developed, and a different camera angle would have revealed whether this fetus is male or female.

FIGURE 2.10 Fetus at 18 weeks
LENART NILSON/SCANPIX

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Figure 2.11: This 18-week-old fetus is clearly sucking its thumb, in much the same way it will as a newborn. The fetus is covered with very fine hair, and a greasy coating protects its skin from its long immersion in liquid.

Figure 2.12: By the 20th week, the fetus spends increasingly more time in a head-down position. The components of facial expressions are present—the fetus can raise its eyebrows, wrinkle its forehead, and move its mouth. As the fetus rapidly puts on weight, the amniotic sac becomes more cramped, leading to a decrease in fetal movements.

FIGURE 2.12 Fetus at 20 weeks
LENART NILSON/SCANPIX
FIGURE 2.13 Fetus at 28 weeks
PETIT FORMAT/NESTLE/SCIENCE SOURCE/PHOTO RESEARCHERS, INC.

Figure 2.13: The 28th week marks the point at which the brain and lungs are sufficiently developed that a fetus born at this time would have a chance of surviving on its own, without medical intervention. The eyes can open, and they move, especially during periods of rapid eye movement (REM) sleep. The auditory system is now functioning, and the fetus hears and reacts to a variety of sounds. At this stage of development, the neural activity of the fetus is very similar to that of a newborn. During the last 3 months of prenatal development, the fetus grows dramatically in size, essentially tripling its weight.

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The typical result of this 9-month period of rapid and remarkable development is a healthy newborn.

Fetal Behavior

As we have noted, the fetus is an active participant in, and contributor to, its own physical and behavioral development. Indeed, the normal formation of organs and muscles depends on fetal activity, and the fetus rehearses the behavioral repertoire it will need at birth.

Movement

Few mothers realize how early their child started moving in the womb. From 5 or 6 weeks after conception, the fetus moves spontaneously, starting with a simple bending of the head and spine that is followed by the onset of increasingly complex movements over the next weeks (De Vries, Visser, & Prechtl, 1982). One of the earliest distinct patterns of movement to emerge (at around 7 weeks) is, remarkably enough, hiccups. Although the reasons for prenatal hiccups are unknown, one recent theory posits that they are essentially a burping reflex, preparing the fetus for eventual nursing by removing air from the stomach and making more room for milk (Howes, 2012).

Developmental psychologist Janet DiPietro is using ultrasound to study the movement patterns of this woman’s fetus.
COURTESTESY JANET DIPIETRO

The fetus also moves its limbs, wiggles its fingers, grasps the umbilical cord, moves its head and eyes, and yawns. Complete changes of position are achieved by a kind of backward somersault. These various movements are initially jerky and uncoordinated but gradually become more integrated. By 12 weeks, most of the movements that will be present at birth have appeared (De Vries et al., 1982), although the mother is still unaware of them.

Later on, when mothers can readily feel the movement of their fetuses, their reports reveal that how much a fetus moves is quite consistent over time: some fetuses are usually very active, whereas others are more sedentary (Eaton & Saudino, 1992). This prenatal continuity extends into the postnatal period: more active fetuses turn out to be more active infants (DiPietro et al., 1998). Furthermore, fetuses that have regular periods of sleep and waking are more likely to have regular sleep times as newborns (DiPietro, Bornstein et al., 2002).

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A particularly important form of fetal movement is swallowing. The fetus drinks amniotic fluid, which passes through its gastrointestinal system. Most of the fluid is then excreted back out into the amniotic sac. One benefit of this activity is that the tongue movements associated with drinking and swallowing promote the normal development of the palate (Walker & Quarles, 1976). In addition, the passage of amniotic fluid through the digestive system helps it to mature properly. Thus, swallowing amniotic fluid prepares the fetus for survival outside the womb.

A second form of fetal movement anticipates the fact that at birth the newborn must start breathing. For that to happen, the lungs and the rest of the respiratory system, including the muscles that move the diaphragm in and out, must be mature and functional. Beginning as early as 10 weeks after conception, the fetus promotes its respiratory readiness by exercising its lungs through “fetal breathing,” moving its chest wall in and out (Nathanielsz, 1994). No air is taken in, of course; rather, small amounts of amniotic fluid are pulled into the lungs and then expelled. Unlike real breathing, which involves an ongoing and consistent pattern of lung activity, fetal breathing is initially infrequent and irregular, but it increases in rate and stability, especially over the third trimester (Govindan et al., 2007).

Behavioral Cycles

“It’s a baby. Federal regulations prohibit our mentioning its race, age, or gender.”
© THE NEW YORKER COLECTION 1996 PETER STEINER FROM CARTOONBANK.COM. ALL RIGHTS RESERVED

Once the fetus begins to move at 5 to 6 weeks, it is in almost constant motion for the next month or so. Then periods of inactivity gradually begin to occur. Rest–activity cycles—bursts of high activity alternating with little or no activity for a few minutes at a time—emerge as early as 10 weeks and become very stable during the second half of pregnancy (Robertson, 1990). In the latter half of the prenatal period, the fetus moves only about 10% to 30% of the time (DiPietro et al., 1998).

Longer-term patterns, including daily (circadian) rhythms, also become apparent, with less activity in the early morning and more activity in the late evening (Arduini, Rizzo, & Romanini, 1995). This confirms the impression of most pregnant women that their fetuses wake up and start doing acrobatics just as they themselves are trying to go to sleep.

Near the end of pregnancy, the fetus spends more than three-fourths of its time in quiet and active sleep states like those of the newborn (James et al., 1995). The active sleep state is characterized by REM, just as it is in infants and adults.

Fetal Experience

There is a popular idea—promoted by everyone from scholars to cartoonists—that we spend our lives longing for the tranquil sanctuary we experienced in our mother’s womb. But is the womb a haven of peace and quiet? Although the uterus and the amniotic fluid buffer the fetus from much of the stimulation impinging on the mother, research has made it clear that the fetus experiences an abundance of sensory stimulation.

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Sight and Touch

Although it is not totally dark inside the womb, the visual experience of the fetus is minimal. The fetus does, however, experience tactile stimulation as a result of its own activity. In the course of moving around, its hands come into contact with other parts of its body: fetuses have been observed not only grasping their umbilical cords but also rubbing their face and sucking their thumbs (Figure 2.11). Indeed, the majority of fetal arm movements during the second half of pregnancy result in contact between their hand and mouth (Myowa-Yamakoshi & Takeshita, 2006). As the fetus grows larger, it bumps against the walls of the uterus increasingly often. By full term, fetuses respond to maternal movements (repeated rocking and swaying), suggesting that their vestibular systems—the sensory apparatus in the inner ear that provides information about movement and balance—is also functioning before birth (Lecanuet & Jacquet, 2002).

Taste

The amniotic fluid contains a variety of flavors (Maurer & Maurer, 1988). The fetus can detect these flavors, and likes some better than others. Indeed, the fetus has a sweet tooth. The first evidence of fetal taste preferences came from a medical study performed more than 60 years ago (described by Gandelman, 1992). A physician named DeSnoo devised an ingenious treatment for women with excessive amounts of amniotic fluid. He injected saccharin into their amniotic fluid, hoping that the fetus would help the mother out by ingesting increased amounts of the sweetened fluid, thereby diminishing the excess. And, in fact, tests of the mothers’ urine showed that the fetuses ingested more amniotic fluid when it had been sweetened, demonstrating that taste sensitivity and flavor preferences exist before birth.

Smell

Amniotic fluid takes on odors from what the mother has eaten (Mennella, Johnson, & Beauchamp, 1995). Obstetricians have long reported that during birth they can smell scents like curry and coffee in the amniotic fluid of women who had recently consumed them. Indeed, human amniotic fluid has been shown to be rich in odorants (although many do not sound very appealing—including those described as being pungently rancid, goaty, or having a “strong fecal note”; Schaal, Orgeur, & Rognon, 1995). Smells can be transmitted through liquid, and amniotic fluid comes into contact with the fetus’s odor receptors through fetal breathing, providing fetuses with the opportunity for olfactory experience. Indeed, as discussed in Box 2.3, rat pups use the familiar scent of their mother’s amniotic fluid to find their mother’s nipples after birth.

Hearing

The fetus of this pregnant woman may be “eavesdropping” on her conversation with her friends.
DIGITAL VISION LT D./SUPERSTOCK

Picture serious scientists hovering over a pregnant woman’s bulging abdomen, ringing bells, striking a gong, clapping blocks of wood together, and even sounding an automobile horn—all to see if her fetus reacts to auditory stimulation. (Remind you of the opening to this chapter?) Such research has demonstrated that external sounds that are audible to the fetus include the voices of people talking to the woman. In addition, the prenatal environment includes many maternal sounds—the mother’s heartbeat, blood pumping through her vascular system, her breathing, her swallowing, and various rude noises made by her digestive system. A particularly prominent and frequent source of sound stimulation is the mother’s voice as she talks, with the clearest aspects being the general rhythm and pitch patterns of her speech.

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The fetus responds to these various sounds from at least the 6th month of pregnancy on. During the last trimester, external noises elicit changes in fetal movements and heart rate (Kisilevsky, Fearon, & Muir, 1998; Lecanuet et al., 1995; Zimmer et al., 1993). By the time fetuses are at term, changes in heart rate patterns suggest that they can distinguish between music and speech played near the mother’s abdomen (Granier-Deferre et al., 2011). The fetus’s heart rate also decelerates briefly when the mother starts speaking (Fifer & Moon, 1995). (Transitory heart-rate deceleration is a sign of interest.) The fetus’s extensive auditory experience with human voices has some lasting effects, as we discuss in the next section.

Fetal Learning

FIGURE 2.14 Habituation Habituation occurs in response to the repeated presentation of a stimulus. As the first stimulus is repeated and becomes familiar, the response to it gradually decreases. When a novel stimulus occurs, the response recovers. The decreased response to the repeated stimulus indicates the formation of memory for it; the increased response to the novel stimulus indicates discrimination of it from the familiar one, as well as a general preference for novelty.

To this point, we have emphasized the impressive behavioral and sensory capabilities of the fetus in the early stages of development. Even more impressive is the extent to which the fetus learns from many of its experiences in the last 3 months of pregnancy, after the central nervous system is adequately developed to support learning.

habituation a simple form of learning that involves a decrease in response to repeated or continued stimulation

Direct evidence for human fetal learning comes from studies of habituation, one of the simplest forms of learning (Thompson & Spencer, 1966). Habituation involves a decrease in response to repeated or continued stimulation (see Figure 2.14). If you shake a rattle beside an infant’s head, the baby will likely turn toward it. At the same time, the infant’s heart rate may slow momentarily, indicating interest. If you repeatedly shake the rattle, however, the head-turning and heart-rate changes will decrease and eventually stop. This decreased response is evidence of learning and memory: the stimulus loses its novelty (and becomes boring) only if the infant remembers the stimulus from one presentation to the next. When a new stimulus occurs, the habituated response recovers (increases). Shaking a bell, for example, may reinstate the head-turning and heart-rate responses. (Developmental psychologists have exploited habituation to study a great variety of topics that you will read about in later chapters.) The earliest time at which fetal habituation has been observed is 30 weeks, indicating that the central nervous system is sufficiently developed at this point for learning and short-term memory to occur (Dirix et al., 2009).

The mother’s voice is probably the most interesting sound frequently available to fetuses. If fetuses can learn something about their mother’s voice prenatally, this could provide them with a running start for learning about other aspects of speech after birth. To test this idea, Kisilevsky and colleagues (2003) tested term fetuses in one of two conditions. Half of the fetuses listened to a recording of their mother reading a poem, played through speakers placed on their mother’s abdomen. The other half listened to recordings of the same poem read by another woman. The researchers found that fetal heart rate increased in response to the mother’s voice, and decreased in response to the other woman’s voice. These findings suggest that the fetuses recognized (and were aroused by) the sound of their own mother’s voice relative to a stranger’s voice. For this to be the case, fetuses must be learning and remembering the sound of their mother’s voice.

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After birth, do newborns remember anything about their fetal experience? The answer is a resounding yes! Like the rat pups discussed in Box 2.3, newborn humans remember the scent of the amniotic fluid in which they lived prenatally. In one set of studies, newborns were presented with two pads, one saturated with their own amniotic fluid and the other saturated with the amniotic fluid of a different baby. With the two pads located on either side of their head, the infants revealed a preference for the scent of their own amniotic fluid by keeping their head oriented longer toward that scent (Marlier, Schaal, & Soussignan, 1998; Varendi, Porter, & Winberg, 2002). These findings extend to specific flavors ingested by the mother. For example, infants whose mothers ate anise (licorice flavor) while they were pregnant preferred the scent of anise at birth, while infants whose mothers did not eat anise showed either a neutral or negative response to its scent (Schaal, Marlier, & Soussignan, 2000).

FIGURE 2.15 Prenatal learning This newborn can control what he gets to listen to. His pacifier is hooked up to a computer, which is in turn connected to an audio player. If the baby sucks in one pattern (predetermined by the researchers), he will hear one recording. If he sucks in a different pattern, he will hear a different recording. Researchers have used this technique to investigate many questions about infant abilities, including the influence of fetal experience on newborn preferences.
MELANIE SPENCE, UNIVERSITY OF TEXAS

Experiences in the womb can lead to long-lasting taste preferences. In one study, pregnant women were asked to drink carrot juice four days a week for three weeks near the end of their pregnancy (Mennella, Jagnow, & Beauchamp, 2001). When tested at around 5½ months of age, their babies reacted more positively to cereal prepared with carrot juice than to the same cereal prepared with water. Thus, the flavor preferences of these babies reflected the influence of their experience in the womb several months earlier. This finding reveals a persistent effect of prenatal learning. Furthermore, it may shed light on the origins and strength of cultural food preferences. A child whose mother ate a lot of chili peppers, ginger, and cumin during pregnancy, for example, might be more favorably disposed to Indian food than would a child whose mother’s diet lacked those flavors.

Along with taste, newborns also remember sounds they heard in the womb. In a classic study, DeCasper and Spence (1986) asked pregnant women to read aloud twice a day from The Cat in the Hat (or another Dr. Seuss book) during the last 6 weeks of their pregnancy. Thus, the women’s fetuses were repeatedly exposed to the same highly rhythmical pattern of speech sounds. The question was whether they would recognize the familiar story after birth. To find out, the researchers tested them as newborns. The infants were fitted with miniature headphones and given a special pacifier to suck on (see Figure 2.15). When the infants sucked in one particular pattern, they heard the familiar story through the headphones, but when they sucked in a different pattern, they heard an unfamiliar story. The babies quickly increased their sucking in the pattern that enabled them to hear the familiar story. Thus, these newborns apparently recognized and preferred the rhythmic patterns from the story they had heard in the womb.

Newborns exhibit numerous additional auditory preferences based on prenatal experience. To begin with, they prefer to listen to their own mother’s voice rather than to the voice of another woman (DeCasper & Fifer, 1980). But how do researchers know that this isn’t due to experience in the hours or days after birth? It turns out that newborns prefer to listen to a version of their mother’s voice that has been filtered to sound the way it did in the womb (Moon & Fifer, 1990; Spence & Freeman, 1996). Finally, newborns would rather listen to the language they heard in the womb than to another language (Mehler et al., 1988; Moon, Cooper, & Fifer, 1993). Newborns whose mothers speak French prefer listening to French over Russian, for example, and this preference is maintained when the speech is filtered to sound the way it sounded in the womb.

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There can be little question that the human fetus is listening and learning. Does this mean that parents-to-be should sign up for programs that promise to “educate your unborn child”? Such programs exhort the mother-to-be to talk to her fetus, read books to it, play music through speakers attached to her abdomen, and so on. Some also urge the father-to-be to speak through a megaphone aimed at his wife’s bulging belly in the hope that the newborn will recognize his voice as well as the mother’s. Is there any point in such exercises?

Probably not. Although it seems possible that hearing Dad’s voice more clearly and more frequently might lead the newborn to prefer it over unfamiliar voices, such a preference develops very quickly after birth anyway. And it is quite clear that some of the advertised advantages of prenatal training would not occur. In the first place, the fetal brain is unlikely to be sufficiently developed to be able to process much about language meaning (after all, even newborn infants can’t learn words). In addition, the liquid environment in the womb—provided by the amniotic fluid—filters out detailed speech sounds, leaving only pitch contours and rhythmic patterns. Brain development aside, this acoustic environment, along with the fetus’s lack of visual access to the external world, would make it impossible for a fetus to learn the meaning of words or any kind of factual knowledge, no matter how much the mother-to-be might read aloud. In short, what the fetus learns about is the mother’s voice and the general patterns of her language—not any specific content. We suspect that the current craze for “prenatal education” will go the way of other ill-conceived attempts to shape early development to adult desires.

Hazards to Prenatal Development

Thus far, our focus has been on the normal course of development before birth. Unfortunately, prenatal development is not always free of error or misfortune. The most dire, and by far the most common, misfortune is spontaneous abortion—commonly referred to as miscarriage. Most miscarriages occur before the woman even knows that she is pregnant. For example, in a Chinese sample, Wang and colleagues (2003) found that approximately one-third of the fetuses did not survive to birth, and that two-thirds of those miscarriages occurred before the pregnancy was clinically detectable. The majority of embryos that are miscarried very early have severe defects, such as a missing chromosome or an extra one, that make further development impossible. In the United States, about 15% of clinically recognized pregnancies end in miscarriage (Rai & Regan, 2006). Across their childbearing years, at least 25% of women—and possibly as many as 50%—experience at least one miscarriage. Few couples realize how common this experience is, making it all the more painful if it happens to them. Yet more agonizing is the experience of the approximately 1% of couples who experience recurrent miscarriages, or the loss of three or more consecutive pregnancies (Rai & Regan, 2006).

For fetuses that survive the danger of miscarriage, there is still a range of factors that can lead to unforeseen negative consequences. Genetic factors, which are the most common, will be discussed in the next chapter. Here, we consider some of the many environmental influences that can have harmful effects on prenatal development.

Environmental Influences

In the spring of 1956, two sisters were brought to a Japanese hospital, delirious and unable to walk. Their parents and doctors were mystified by the sudden deterioration in the girls, described as having been “the brightest, most vibrant, cutest kids you could imagine.” The mystery intensified as more children and adults developed nearly identical symptoms. The discovery that all the patients were from the small coastal town of Minamata suggested a common cause for what was referred to as the “strange disease” (Newland & Rasmussen, 2003; Smith & Smith, 1975).

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Victims of “Minamata disease” include individuals who were exposed to methylmercury prenatally.
MICHAEL S. YAMASHITA/CORBIS

That cause was eventually traced to the tons of mercury that had been dumped into Minamata Bay by a local petrochemical and plastics factory. For years, the residents of Minamata had been catching and consuming fish that had absorbed mercury from the polluted waters of the bay. By 1993, more than 2000 children and adults had been diagnosed with what had come to be known as “Minamata disease”—methylmercury poisoning (Harada, 1995). At least 40 children had been poisoned prenatally by mercury in the fish eaten by their pregnant mothers and were born with cerebral palsy, intellectual disabilities, and a host of other neurological disorders.

teratogen an external agent that can cause damage or death during prenatal development

The tragedy of Minamata Bay provided some of the first clear evidence of the seriously detrimental impact that environmental factors can have on prenatal development. As you will see, a vast array of environmental agents, called teratogens, have the potential to harm the fetus. The resulting damage ranges from relatively mild and easily corrected problems to fetal death.

sensitive period the period of time during which a developing organism is most sensitive to the effects of external factors; prenatally, the sensitive period is when the fetus is maximally sensitive to the harmful effects of teratogens

A crucial factor in the severity of the effects of potential teratogens is timing (one of the basic developmental principles discussed in Chapter 1). Many teratogens cause damage only if they are present during a sensitive period in prenatal development. The major organ systems are most vulnerable to damage at the time when their basic structures are being formed. Because the timing is different for each system, the sensitive periods are different for each system, as shown in Figure 2.16.

FIGURE 2.16 Sensitive periods of prenatal development The most sensitive or critical period of prenatal development is the embryonic period. During the first 2 weeks, before implantation in the uterus, the zygote is generally not susceptible to environmental factors. Every major organ system of the body undergoes all or a major part of its development between the 3rd and the 9th week. The dark green portions of the bars in the figure denote the times of most rapid development when major defects originate. The light green portions indicate periods of continued but less rapid development when minor defects may occur. (Adapted from Moore & Persaud, 1993)

There is no more dramatic or straightforward illustration of the importance of timing than the birth outcomes related to the drug thalidomide in the early 1960s. Thalidomide was prescribed to treat morning sickness (among other things), and was considered to be so safe that it was sold over the counter. At the time, it was believed that such medications would not cross the placental barrier. However, many pregnant women who took this new, presumably safe sedative gave birth to babies with major limb deformities; some babies were born with no arms and with flipperlike hands growing out of their shoulders. In a striking illustration of sensitive period effects, serious defects occurred only if the pregnant woman took the drug between the 4th and 6th week after conception, the time when her fetus’s limbs were emerging and developing (look again at Figure 2.6 to Figure 2.13). Taking thalidomide either before the limbs started to develop or after they were basically formed had no harmful effect.

As you can see in Figure 2.16, the sensitive periods for many organ systems—and hence the time when the most significant teratogenic damage can result from something the mother does or experiences—occur before the woman might realize she is pregnant. Because a substantial number of pregnancies are unplanned, sexually active people of childbearing age need to be aware of behaviors that could compromise the health of a child they might conceive.

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dose–response relation a relation in which the effect of exposure to an element increases with the extent of exposure (prenatally, the more exposure a fetus has to a potential teratogen, the more severe its effect is likely to be)

Another crucial factor influencing the severity of teratogenic effects is the amount and length of exposure. Most teratogens show a dose–response relation: the greater the fetus’s exposure to a potential teratogen, the more likely it is that the fetus will suffer damage and the more severe any damage is likely to be.

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Avoiding environmental agents that have teratogenic effects is complicated by the fact that they often cannot be readily identified. One reason is that environmental risk factors frequently occur in combination, making it difficult to separate out their effects. For families living in urban poverty, for example, it is hard to tease apart the effects of poor maternal diet, exposure to airborne pollution, inadequate prenatal care, and psychological stress resulting from underemployment, single parenthood, and living in crime-ridden neighborhoods.

This young artist was damaged while in the womb because his mother took the drug thalidomide. She must have taken the drug in the second month of her pregnancy, the time when the arm buds develop—an unfortunate example providing clear evidence of the importance of timing in how environmental agents can affect the developing fetus.
PAUL FIEVEZ/BIPS/GETY IMAGES

Furthermore, the presence of multiple risk factors can have a cumulative impact. For example, in the case of marginal prenatal nutrition, the fetus’s metabolism adjusts to the level of nutritional deficiency experienced in the womb and does not reset itself after birth. In a postnatal environment with abundant opportunities for caloric intake, this sets the stage for the development of overweight and obesity. Such belated emergence of effects of prenatal experience is referred to as fetal programming, because experiences during the prenatal period “program the physiological set points that will govern physiology in adulthood” (Coe & Lubach, 2008).

The effects of teratogens can also vary according to individual differences in genetic susceptibility (probably in both the mother and the fetus). Thus, a substance that is harmless to most people may trigger problems in a minority of individuals, whose genes predispose them to be affected by it.

Finally, identifying teratogens is further complicated by the existence of sleeper effects, in which the impact of a given agent may not be apparent for many years. For example, between the 1940s and 1960s, the hormone diethylstilbestrol (DES) was commonly used to prevent miscarriage and had no apparent ill effects on babies born to women who had taken it. However, in adolescence and adulthood, these offspring turned out to have elevated rates of cervical and testicular cancers.

An enormous number of potential teratogens have been identified, but we will focus only on some of the most common ones, emphasizing in particular those that are related to the behavior of the pregnant woman. Table 2.2 includes the agents discussed in the text as well as several additional ones, but you should be aware that there are numerous other agents known to be, or suspected of being, hazardous to prenatal development.

Legal drugs Although many prescription and over-the-counter drugs are perfectly safe for pregnant women, some are not. Pregnant women (and women who have reason to think they might soon become pregnant) should take drugs only under the supervision of a physician. This issue can become particularly acute in the face of public health emergencies like the 2009 H1N1 (swine flu) pandemic, during which even some physicians were confused about the appropriateness of common medications for pregnant women, including the influenza vaccine and acetaminophen (Tylenol) (Rasmussen, 2012). Other prescription drugs that are in common use by women of childbearing age, such the acne medication isotretinoin (Accutane), are known human teratogens that cause severe birth defects or fetal death. Indeed, because of the unambiguous relationship between Accutane and birth defects, physicians require women to comply with multiple contraceptive measures and ongoing pregnancy tests before prescribing the drug.

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The two legal “drugs” that wreak the most havoc on fetal development are cigarettes (nicotine) and alcohol. Because the use of these substances represents a lifestyle choice rather than a medical remedy for a specific condition (like flu shots, antiseizure medications, or Accutane), their effects are particularly widespread.

CIGARETTE SMOKING We all know that smoking is unhealthy for the smoker, and there is abundant evidence that it is not good for the smoker’s fetus, either. When a pregnant woman smokes a cigarette, she gets less oxygen, and so does her fetus. Indeed, the fetus makes fewer breathing movements while its mother is smoking. In addition, the fetuses of smokers metabolize some of the cancer-causing agents contained in tobacco. And because the mother-to-be inhales cigarette gases when someone else is smoking nearby, secondhand smoke has an indirect effect on fetal oxygen.

This woman is endangering the health of her fetus.
JUAN COLADO/GETY IMAGES

The main developmental consequences of maternal smoking are slowed fetal growth and low birth weight, both of which compromise the health of the newborn. In addition, evidence suggests that smoking may be linked to increased risk of sudden infant death syndrome (SIDS) (discussed in Box 2.4) and a variety of other problems, including lower IQ, hearing deficits, and cancer.

In spite of the well-documented negative effects of maternal smoking on fetal development, it is estimated that approximately 1 in 10 women in the United States smokes during pregnancy (Centers for Disease Control, 2009; Child Trends, 2012). For women who manage to quit smoking during pregnancy, the relapse rate is high after they give birth; roughly half begin smoking again within the first 6 months after their baby is born. Taken together, these data show that many infants are exposed to a known teratogen before birth, and numerous additional infants are exposed to a known health hazard after birth. Given that the negative effects of maternal smoking on fetal development are well publicized, you may not find it surprising that mothers who nevertheless smoke during pregnancy are less sensitive and less warm in interactions with their young infants (Schuetze, Eiden, & Dombkowski, 2006).

ALCOHOL Alcohol is currently “the most common human teratogen” (Ramadoss et al., 2008). Maternal alcohol use is the leading cause of fetal brain injury and is generally considered to be the most preventable cause. According to data collected between 2005 and 2010, approximately 7.6% of women used alcohol during their pregnancies (Centers for Disease Control, 2012). Surprisingly, women who are White, older than 35 years, and employed are more likely to drink during pregnancy than are women who are non-White, younger than 24 years, and unemployed. This statistic reverses the more typical pattern of maternal teratogen exposure, which tends to predominate among expectant mothers with fewer economic and social resources.

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Box 2.4: applications: FACE UP TO WAKE UP

sudden infant death syndrome (SIDS) the sudden, unexpected death of an infant less than 1 year of age that has no identifiable cause

For parents, nothing is more terrifying to contemplate than the death of their child. New parents are especially frightened by the specter of sudden infant death syndrome (SIDS). SIDS refers to the sudden, unexpected, and unexplained death of an infant younger than 1 year. The most common SIDS scenario is that an apparently healthy baby, usually between 2 and 5 months of age, is put to bed for the night and found dead in the morning. In the United States, the incidence of SIDS is 56 per 10,000 live births, making it the leading cause of infant mortality between 28 days and 1 year of age (Task Force on Sudden Infant Death Syndrome, 2011). African American and Native American infants are most likely to die from SIDS, whereas Hispanic American and Asian American infants are least likely to die from SIDS. These patterns suggest cultural differences in parenting that might protect some infants from SIDS.

“Face Up to Wake Up.” The parents of this infant are following the good advice of the foundation dedicated to lowering the incidence of SIDS worldwide. Since the inauguration of this campaign, SIDS in the United States has declined to half its previous rate (Task Force on Sudden Infant Death Syndrome, 2011).
SINGHSOMEN/DREAMSTIME.COM

The causes of SIDS are still not well understood. One hypothesis is that SIDS may involve an inadequate reflexive response to respiratory occlusion—that is, an inability to remove or move away from something covering the nose and mouth (Lipsitt, 2003). Infants may be particularly vulnerable to SIDS between 2 and 5 months of age because that is when they are making a transition from neonatal reflexes under the control of lower parts of the brain (the brainstem) to deliberate, learned behaviors mediated by higher brain areas (cerebral cortex). A waning respiratory occlusion reflex during this transition period may make infants less able to effectively pull their head away from a smothering pillow or to push a blanket away from their face.

In spite of the lack of certainty about the causes of SIDS, researchers have identified several steps that parents can take to decrease the risk to their baby. The most important one is putting infants to sleep on their back, reducing the possibility of anything obstructing their breathing. Sleeping on the stomach increases the risk of SIDS more than any other single factor (e.g., Willinger, 1995). (With respect to the cultural differences in the incidence of SIDS mentioned above, it is significant that Hispanic American parents are the most likely to put their infants to sleep on their back [73%], and African Americans the least likely to do so [53%].) A campaign encouraging parents to put their infants to sleep on their back—the “back to sleep” movement—has contributed to a dramatic reduction in the number of SIDS victims.

Second, to lower the risk of SIDS, parents should not smoke. If they do smoke, they should not smoke around the baby. Infants whose mothers smoke during pregnancy and/or after the baby’s birth are more than 3½ times more likely to succumb to SIDS than are babies who are not exposed to smokers in their home (Anderson, Johnson, & Batal, 2005).

Third, babies should sleep on a firm mattress with no pillow or crib bumpers. Soft bedding can trap air around the infant’s face, causing the baby to breathe in his or her own carbon dioxide instead of oxygen.

Fourth, infants should not be wrapped in lots of blankets or clothes. Being overly warm is associated with SIDS.

Fifth, infants who are breastfed are less likely to succumb to SIDS (e.g., Hauck et al., 2011). Why would breastfeeding protect infants from SIDS? One possible reason is that breastfed infants are more easily aroused from sleep than formula-fed infants, and thus may more easily detect when their airflow is interrupted (Horne et al., 2004).

One unanticipated consequence of the “back to sleep” movement has been that North American infants are now beginning to crawl slightly later than those in previous generations, presumably because of reduced opportunity to strengthen their muscles by pushing up off their mattress. Parents are encouraged to give their babies supervised “tummy time” to exercise their muscles during the day.

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Women who use alcohol before becoming pregnant (about half of women of childbearing age) are most likely to continue using alcohol during pregnancy. In part, this is due to the fact that in the United States, 40% of women do not realize that they are pregnant until after the fourth week of gestation, when they have missed a menstruation cycle. As we have seen, those early weeks are a crucial period in fetal development.

fetal alcohol spectrum disorder (FASD) the harmful effects of maternal alcohol consumption on a developing fetus. Fetal alcohol syndrome (FAS) involves a range of effects, including facial deformities, mental retardation, attention problems, hyperactivity, and other defects. Fetal alcohol effects (FAE) is a term used for individuals who show some, but not all, of the standard effects of FAS.

When a pregnant woman drinks, the alcohol in her blood crosses the placenta into both the fetus’s bloodstream and the amniotic fluid. Thus, the fetus gets alcohol directly in its bloodstream, and indirectly by drinking an amniotic-fluid cocktail. Concentrations of alcohol in the blood of mother and fetus quickly equalize, but the fetus has less ability to metabolize and remove alcohol from its blood, so it remains in the fetus’s system longer. Immediate behavioral effects on the fetus include altered activity levels and abnormal startle reflexes (Little, Hepper, & Dornan, 2002).

In the long run, maternal drinking can result in fetal alcohol spectrum disorder(FASD) (Sokol et al., 2003), which comprises a continuum of alcohol-related birth defects. Babies born to alcoholic women often exhibit a condition known as fetal alcohol syndrome (FAS) (Jacobson & Jacobson, 2002; Jones & Smith, 1973; Streissguth, 2001; Streissguth et al., 1993). The most obvious symptoms of FAS are facial deformities like those shown in Figure 2.17. Other forms of FAS can include varying degrees of intellectual disability, attention problems, and hyperactivity. Many children who were prenatally exposed to alcohol and show similar but fewer symptoms are diagnosed with fetal alcohol effects (FAE) (Mattson et al., 1998).

FIGURE 2.17 Facial Features of FAS These two children display the three primary diagnostic facial features of fetal alcohol syndrome: small eyes (as measured across); the absence of, or flattening of, the vertical groove between the nose and the upper lip (smooth philtrum); and a thin upper lip. It appears that the more pronounced these features are in an affected child, the greater the likelihood that the child experienced prenatal brain damage. Roughly 1 in 1000 infants born in the United States has FAS.
SUSAN ASTLEY, PHD, UNIVERSITY OF WASHINGTON
RICK’S PHOTOGRAPHY/SHUTTERSTOCK

Even moderate drinking during pregnancy (i.e., less than one drink per day) can have both short- and long-term negative effects on development. So can occasional drinking if it involves binge drinking (more than five drinks per episode) (e.g., Hunt et al., 1995; Sokol et al., 2003). And according to an analysis of self-report data from 2006 through 2010, 1.4% of pregnant women in the United States engage in at least one incident of binge drinking during their pregnancy (Centers for Disease Control, 2012).

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Given the potential outcomes and the fact that no one knows whether there is a safe level of alcohol consumption for a pregnant woman, the best approach for expectant mothers is to avoid alcohol altogether.

Illegal drugs In the United States, the use of illegal drugs during pregnancy ranges from a low of 3.1% among Hispanic women to a high of 7.7% among non-Hispanic Black women (National Survey on Drug Use and Health, 2012). Almost all commonly abused illegal drugs have been shown to be, or are suspected of being, dangerous for prenatal development. It has proved difficult to pin down exactly how dangerous particular drugs are, however, because pregnant women who use one illegal substance often use others, along with smoking cigarettes and drinking alcohol (Frank et al., 2001; Lester, 1998; Smith et al., 2006).

Prenatal exposure to marijuana, the illegal substance most commonly used by women of reproductive age in the United States, is suspected of affecting memory, learning, and visual skills after birth (Fried & Smith, 2001; Mereu et al., 2003). Cocaine in its various forms is the second most common illegal drug abused by young American women (Substance Abuse and Mental Health Services Administration, 2011). Although some early reports of devastating effects from cocaine use during pregnancy turned out to be exaggerated, such use has been associated with fetal growth retardation and premature birth (Hawley & Disney, 1992; Singer et al., 2002). In addition, infants who endured prenatal exposure to cocaine have impaired ability to regulate arousal and attention (e.g., DiPietro et al., 1995; Lewkowicz, Karmel, & Gardner, 1998). Especially distressing is the case of newborns born to coke-addicted mothers, because they have to go through withdrawal just like a reforming addict (Kuschel, 2007).

Longitudinal studies of the development of cocaine-exposed children have revealed persistent, although sometimes subtle, cognitive and social deficits (Lester, 1998). These deficits can be ameliorated to some degree, as suggested by improved outcomes among affected children who were adopted into supportive middle-class families (Koren et al., 1998).

FIGURE 2.18 Hearing loss in children whose mothers worked in a noisy factory while pregnant The greater the noise exposure a pregnant woman experienced, the greater the hearing impairment of her child.
LALANDE, HÉTU, & LAMBERT, 1986

Environmental pollutants The bodies and bloodstreams of most Americans (including women of childbearing age) contain a noxious mix of toxic metals, synthetic hormones, and various ingredients of plastics, pesticides, and herbicides that can be teratogenic (Moore, 2003). Echoing the story of Minamata disease, evidence has accumulated that mothers whose diet was high in Lake Michigan fish with high levels of polychlorinated biphenyls (PCBs) had newborns with small heads. The children with the highest prenatal exposure to PCBs had slightly lower IQ scores as long as 11 years later (Jacobson & Jacobson, 1996; Jacobson et al., 1992). In China, the rapid modernization that has led to economic success has also taken a toll on health in general, and has led to a dramatic increase in pollution-related birth defects due to the unregulated burning of coal, water pollution, and pesticide use (e.g., Ren et al., 2011).

Occupational hazards Many women have jobs that bring them into contact with a variety of environmental elements that are potentially hazardous to prenatal development. Tollbooth collectors, for example, are exposed to high levels of automobile exhaust; farmers, to pesticides; and factory workers, to numerous chemicals. As Figure 2.18 shows, even noise pollution can negatively affect fetal development. Employers and employees alike are grappling with how best to protect pregnant women from potential teratogens without subjecting them to job discrimination.

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Maternal Factors

Because the mother-to-be provides the most immediate environment for her fetus, some of her characteristics can affect prenatal development. These characteristics include age, nutritional status, health, and stress level.

Age A pregnant woman’s age is related to the outcome of her pregnancy. Infants born to girls 15 years or younger are three to four times more likely to die before their first birthday than are those born to mothers who are between 23 and 29 (Phipps, Blume, & DeMonner, 2002). However, the rate of teenage pregnancy has declined substantially in recent years, and in 2010, the birth rate for teenagers fell to the lowest recorded level in the United States (34 births per 1000 females younger than 20; Hamilton, Martin, & Ventura, 2011).

A different age-related cause for concern has to do with the increasing age of first-time mothers. In recent decades, many women have chosen to wait until their 30s or 40s to have children. At the same time, techniques to treat infertility have continued to improve, increasing the likelihood of conception for older parents. Older mothers are at greater risk for many negative outcomes for themselves and their fetus, including fetal chromosomal abnormalities (see Chapter 3) and birth complications.

Nutrition The fetus depends on its mother for all its nutritional requirements. If a pregnant woman has an inadequate diet, her unborn child may also be nutritionally deprived (Pollitt et al., 1996). An inadequate supply of specific nutrients or vitamins can have dramatic consequences. For example, women who get too little folic acid (a form of B vitamin) are at high risk for having an infant with a neural-tube defect such as spina bifida (see Figure 2.5). General malnutrition affects the growth of the fetal brain: newborns who received inadequate nutrients while in the womb tend to have smaller brains containing fewer brain cells than do well-nourished newborns.

These poor parents in Bolivia are worrying about how they are going to feed their children—a situation all too common throughout the world.
JAVIER TENIENTE/COVER/GETY IMAGES

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Because malnutrition is more common in impoverished families, it often coincides with the host of other risk factors associated with poverty, making it difficult to isolate its effects on prenatal development (Lozoff, 1989; Sigman, 1995). However, one unique study of development in very extreme circumstances made it possible to assess certain effects of malnutrition independent of socioeconomic status (Stein et al., 1975). In parts of Holland during World War II, people of all income and education levels suffered severe famine. Later, the health records of those Dutch women who had been pregnant during this time of general malnourishment were examined. Their babies were, on average, underweight at birth, but the severity of effects depended on how early in their pregnancy the women had become malnourished. Those who became malnourished only in the last few months of pregnancy tended to have slightly underweight babies with relatively small heads. However, those whose malnutrition started early in their pregnancy often had very small babies with serious physical defects.

Disease Although most maternal illnesses that occur during a pregnancy have no impact on the fetus, some do. For example, if contracted early in pregnancy, rubella (also called the 3-day measles) can have devastating developmental effects, including major malformations, deafness, blindness, and intellectual disabilities. Any woman of childbearing age who does not have immunities against rubella should be vaccinated before becoming pregnant.

Sexually transmitted diseases (STDs) that have become increasingly common throughout the world are also quite hazardous to the fetus. Cytomegalovirus, a type of herpes virus that is present in 50% to 80% of the adult population in the United States, is currently the most common cause of congenital infection (1 of every 150 infants). It can damage the fetus’s central nervous system and cause a variety of other serious defects. Genital herpes can also be very dangerous: if the infant comes into contact with active herpes lesions in the birth canal, blindness or even death can result. HIV infection is sometimes passed to the fetus in the womb or during birth, but the majority of infants born to women who are HIV-positive or have AIDS do not become infected themselves. HIV can also be transmitted through breast milk after birth, but recent research suggests that breast milk contains a carbohydrate that may actually protect infants from HIV infection (Bode et al., 2012).

Evidence has been accumulating for effects of maternal illness on the development of psychopathology later in life. For example, the incidence of schizophrenia is higher for individuals whose mothers had influenza (flu) during the first trimester of pregnancy (Brown et al., 2004). Maternal flu may interact with genetic or other factors to lead to mental illness.

Prenatal exercise classes, as well as yoga or meditation classes, may help reduce pregnancy-related stress.
BUBBLES PHOTOLIBRARY/ALAMY

Maternal emotional state For centuries, people have believed that a woman’s emotions can affect her fetus. This view is now supported by research suggesting that maternal stress can have negative consequences for development (DiPietro, 2012). For example, the fetuses of women who reported higher levels of stress during pregnancy were more physically active throughout their gestation than were the fetuses of women who felt less stressed (DiPietro, Hilton et al., 2002). This increased activity is likely related to hormones, including adrenaline and cortisol, that the mother secretes in response to stress (Relier, 2001). Such effects can continue after birth. In a study that involved more than 7000 pregnant women and their infants, maternal anxiety and depression during pregnancy were assessed. The higher the level of distress the pregnant women reported, the higher the incidence of behavior problems in their children at 4 years of age—including hyperactivity and inattention in boys, conduct problems in girls, and emotional problems in both boys and girls (O’Connor et al., 2002). Findings such as these, linking prenatal maternal stress to postnatal behavior problems, are likely to also be mediated by increased levels of maternal hormones, such as cortisol, that are elicited by stress (Susman et al., 2001; Susman, 2006).

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Like other types of teratogens, it is difficult to tease apart the specific effects of maternal stress from other factors that often co-occur with stress; for example, expectant mothers who are stressed during pregnancy are likely to still be stressed after giving birth. That said, the increased popularity of prenatal yoga and meditation classes may point to ways in which pregnancy-related stress may be reduced, with potential benefits for both mother and fetus.

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The most rapid period of development starts at conception, with the union of egg and sperm, and continues for roughly 9 months, divided into three developmental periods: germinal, embryonic, and fetal. The processes through which prenatal development occurs include cell division, cell migration, cell differentiation, and cell death. Every major organ system undergoes all or a substantial part of its development between the 3rd and 8th week following conception, making this a sensitive period for potential damage from environmental hazards.

Scientists have learned an enormous amount about the behavior and experience of the developing organism, which begins to move at 5 to 6 weeks after conception. Some behaviors of the fetus contribute to its development, including swallowing amniotic fluid and making breathing motions. The fetus has relatively rich sensory experience from stimulation both within and outside the womb, and this experience is the basis for fetal learning. Some effects of fetal learning after birth have been shown to be persistent.

Many environmental agents can have a negative impact on prenatal development. The most common teratogens in the United States are cigarette smoking, alcohol consumption, and environmental pollution. Maternal factors (malnutrition, illness, stress, and so forth) can also cause problems for the developing fetus and child. Timing is crucial for exposure to many teratogens; the severity of effects is also related to the amount and length of exposure, as well as to the number of different negative factors with which a fetus has to contend.