Prenatal Development

The most dramatic and extensive transformation of one’s entire life occurs before birth. To make it easier to study, prenatal development is often divided into three main periods. The first two weeks are called the germinal period; the third through the eighth week is the embryonic period; from then until birth is the fetal period. (Alternative terms are presented in Table 4.1.)

Germinal: The First 14 Days

You learned in Chapter 3 that the one-celled zygote duplicates and multiplies. Soon after the 16-cell stage, differentiation begins as those early cells take on distinct characteristics and gravitate toward particular locations.

First Stages of the Germinal Period The original zygote as it divides into (a) two cells, (b) four cells, and (c) eight cells. Occasionally at this early stage, the cells separate completely, forming the beginning of monozygotic twins, quadruplets, or octuplets.

ALL PHOTOS: ANATOMICAL TRAVELOGUE/SCIENCE SOURCE

About a week after conception, the cell mass, now called a blastocyst, forms two distinct parts—a shell that will become the placenta and a nucleus that will become the embryo.

The first task of the outer cells is to achieve implantation—that is, to embed themselves in the nurturing lining of the uterus (see Figure 4.1). This process is far from automatic; about half of natural conceptions and an even larger proportion of in vitro conceptions never implant (see Table 4.2): Most new life ends before an embryo begins (Sadler, 2010).

The Most Dangerous Journey In the first 10 days after conception, the organism does not increase in size because it is not yet nourished by the mother. However, the number of cells increases rapidly as the organism prepares for implantation, which occurs successfully not quite half of the time.

Embryo: From the Third Through the Eighth Week

The start of the third week after conception initiates the embryonic period, during which the formless mass of cells becomes a distinct being—not yet recognizably human but worthy of a new name, embryo. (The word embryo is often used loosely, but each stage of development has a particular name; here, embryo refers to the developing human from day 14 to day 56.)

The Embryonic Period (a) At 4 weeks past conception, the embryo is only about 1/8 inch (3 millimeters) long, but already the head has taken shape. (b) By 7 weeks, the organism is somewhat less than an inch (2 centimeters) long. Eyes, nose, the digestive system, and even the first stage of toe formation can be seen.

OMIKRON/SCIENCE SOURCE
PETIT FORMAT/SCIENCE SOURCE

First, a thin line (called the primitive streak) appears down the middle of the embryo; it will become the neural tube 22 days after conception and eventually develop into the central nervous system (the brain and spinal column). The head appears in the fourth week, as eyes, ears, nose, and mouth start to form. Also in the fourth week, a minuscule blood vessel that will become the heart begins to pulsate.

By the fifth week, buds that will become arms and legs emerge. The upper arms and then forearms, palms, and webbed fingers grow. Legs, knees, feet, and webbed toes, in that order, are apparent a few days later, each having the beginning of a skeletal structure. Then, 52 and 54 days after conception, respectively, the fingers and toes separate (Sadler, 2010).

As you can see, prenatally, the head develops first, in a cephalocaudal (literally, “head-to-tail”) pattern, and the extremities form last, in a proximodistal (literally, “near-to-far”) pattern. At the end of the eighth week after conception (56 days), the embryo weighs just one-thirtieth of an ounce (1 gram) and is about 1 inch (2½ centimeters) long. It has all the basic organs and body parts (except sex organs) of a human being, including elbows and knees. It moves frequently, about 150 times per hour, but this movement is random and imperceptible.

Fetus: From the Ninth Week Until Birth

The organism is called a fetus from the beginning of the ninth week after conception until birth. The fetal period encompasses dramatic change, from a tiny, sexless creature smaller than the final joint of your thumb to a boy or girl about 20 inches (51 centimeters) long.

If the 23rd chromosomes are XY, the SRY gene on the Y triggers the development of male sexual organs. Otherwise, female organs develop. The male fetus experiences a rush of the hormone testosterone, affecting many structures and connections in the brain (Filová et al., 2013). Of course, the range of brain and behavioral variations among males and among females is greater than the variations between the average man and woman. Nonetheless, some neurological sex differences begin early in prenatal development.

By the end of the third month, the sex organs may be visible via ultrasound (in a sonogram), which is similar to an X-ray but uses sound waves instead of radiation. The 3-month-old fetus weighs about 3 ounces (87 grams) and is about 3 inches (7.5 centimeters) long. Early prenatal growth is very rapid, with considerable variation, especially in body weight. The numbers just given—3 months, 3 ounces, 3 inches—are rounded off for easy recollection. (Metric measures—100 days, 100 grams, 100 millimeters—are similarly imprecise yet useful.)

There’s Your Baby For many parents, their first glimpse of their future child is an ultrasound image. This is Alice, 63 days before birth.

ROBIN MORGAN

In the fourth, fifth, and sixth months, the heartbeat becomes stronger. Digestive and excretory systems develop. Fingernails, toenails, and buds for teeth form, and hair grows (including eyelashes). The brain increases about six times in size and develops many new neurons (neurogenesis) and synapses (synaptogenesis). Indeed, up to half a million brain cells per minute are created at peak growth during mid-pregnancy (Dowling, 2004). Following the proximodistal sequence, first the brain stem above the back of the neck, then the midbrain, and finally the cortex develop and connect.

Brain development occurs in every prenatal month, but these middle three months may be the most crucial (Johnson, 2010). Brain growth is critical at this point because the entire central nervous system becomes responsive during mid-pregnancy, beginning to regulate basic body functions such as breathing and sucking. Advances in neurological functioning allow the fetus to reach the age of viability, when a preterm newborn can survive.

With intensive medical care, some babies survive at 22 weeks past conception, although many hospitals worldwide do not routinely initiate intensive care unless the fetus is at least 25 weeks old. The age of viability decreased dramatically in the twentieth century, but it now seems stuck at about 22 weeks (Pignotti, 2010) because even the most advanced technology cannot maintain life without some brain response. (Reports of survivors born before 22 weeks are suspect because the date of conception is unknown.)

As the brain matures, the organs of the body begin to work in harmony. The heart beats faster during activity; fetal movement as well as heart rate quiet down during rest (not necessarily when the mother wants to sleep).

Viability This fetus is in mid-pregnancy, a few weeks shy of viability. As you can see, the body is completely formed. Unseen is the extent of brain and lung development; it will take at least another month for these organs to become sufficiently mature to allow for survival.

LENNART NILSSON/SCANPIX

Reaching viability simply means that life outside the womb is possible. Each day of the final three months improves the odds, not only of survival but also of life without disability (Iacovidou et al., 2010). (More on viability is presented later in this chapter.)

A preterm infant born in the seventh month is a tiny creature requiring intensive care for each gram of nourishment and every shallow breath. By contrast, after nine months or so, the typical full-term newborn is ready to thrive at home on mother’s milk—no expert help, oxygenated air, or special feeding required. For thousands of years, that is how humans survived: We would not be alive if any of our ancestors had required intense newborn care.

The critical difference between life and death, or between a fragile preterm newborn and a robust one, is maturation of the neurological, respiratory, and cardiovascular systems. In the final three months of prenatal life, the lungs begin to expand and contract, and breathing muscles are exercised as the fetus swallows and spits out amniotic fluid. The valves of the heart go through a final maturation, as do the arteries and veins throughout the body. Among other things, this helps to prevent “brain bleeds,” one of the hazards of preterm birth in which paper-thin blood vessels in the skull collapse.

The fetus usually gains at least 4½ pounds (2.1 kilograms) in the third trimester, increasing to almost 7½ pounds (about 3.4 kilograms) at birth. By full term, human brain growth is so extensive that the cortex (the brain’s advanced outer layers) forms several folds in order to fit into the skull (see Figure 4.2). Although some large mammals (whales, for instance) have bigger brains than humans, no other creature needs as many folds because, relative to size, the human cortex contains much more material than the brains of nonhumans.

Prenatal Growth of the Brain Just 25 days after conception (a), the central nervous system is already evident. The brain looks distinctly human by day 100 (c). By the 28th week of gestation (e), at the very time brain activity begins, the various sections of the brain are recognizable. When the fetus is full term (f), all the parts of the brain, including the cortex (the outer layers), are formed, folding over one another and becoming more convoluted, or wrinkled, as the number of brain cells increases.

Source: Adapted from Cowan, 1997, p. 116.

One of the Tiniest Rumaisa Rahman was born after 26 weeks and 6 days, weighing only 8.6 ounces (244 grams). Nevertheless, she has a good chance of living a full, normal life. Rumaisa gained 5 pounds (2,270 grams) in the hospital and then, 6 months after her birth, went home. Her twin sister, Hiba, who weighed 1.3 pounds (590 grams) at birth, had gone home two months earlier. At their one-year birthday, the twins seemed normal, with Rumaisa weighing 15 pounds (6,800 grams) and Hiba 17 (7,711 grams) (CBS News, 2005).

LOYOLA UNIVERSITY HEALTH SYSTEM, HO/AP PHOTO

Can He Hear? A fetus, just about at the age of viability, is shown fingering his ear. Such gestures are probably random; but yes, he can hear.

N. BROMHALL/SCIENCE SOURCE

The relationship between mother and child intensifies during the final three months as the fetus’s size and movement make the pregnant woman very aware of it. In turn, her sounds, the tastes of her food (via amniotic fluid), and her behavior patterns become part of fetal consciousness.

Auditory communication from mother to child begins at the 28th week and improves each week as fetal hearing (or newborn hearing if a baby is born early) becomes more acute (Bisiacchi et al., 2009). The fetus startles and kicks at loud noises, listens to the mother’s heartbeat and voice, and is comforted by rhythmic music and movement, such as when the mother sings as she walks. If the mother is fearful or anxious, the fetal heart beats faster and body movements increase (DiPietro et al., 2002).