The most dramatic and extensive transformation of 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.)

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

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, 2012).

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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.)

First, a thin line (called the primitive streak) appears down the middle of the embryo; it will become the neural tube between 20 and 27 days after conception and eventually develop into the central nervous system (the brain and spinal column) (Stiles & Jernigan, 2010). 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, emerge 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, 2012).

As you can see, prenatally, the head develops first, in a cephalo-caudal (literally, “head-to-tail”) pattern, and the extremities form last, in a proximo-distal (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 imperceptible and random; it will be many months before deliberate movement occurs.

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.

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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, 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.)

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, mid-pregnancy is the peak time for creation of new brain cells. Before this, the cortex had been smooth, but now the brain begins to have the folds and wrinkles that allow a human brain to be far larger and more complex than the brains of other animals (Stiles & Jernigan, 2010). Following the proximo-distal 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 are especially crucial (Johnson, 2011). 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 at the end of this trimester allow the fetus to reach the age of viability, the point of development when a fetus born far too early can become a baby who is able to survive.

Survival is far from automatic; almost all 22-week-old newborns who were born a few decades ago died. Currently, if the fetus has been developing well, the mother is healthy, and birth occurs in an advanced neonatal unit, some very immature babies survive.

In Japan, with excellent neonatal care, 20 percent of 22-week-old newborns survive without major neurological impairment (Ishii et al., 2013). In developing nations, such small babies never live, and even in developed nations, many hospitals do not routinely initiate intensive care unless the fetus is at least 25 weeks old.

Reaching viability simply means that life outside the womb is possible. Many babies born between 22 and 24 weeks die, and survivors born before 27 weeks often develop slowly because they have missed some essential brain development in the uterus (Månsson & Stjernqvist, 2014). Each day of the final three months improves the odds not only of survival but also of a healthy life and normal cognition. (More on preterm birth appears later in this chapter.) Many aspects of prenatal life are awe-inspiring; the fact that an ordinary woman provides a far better home for a fetus than the most advanced medical technology can attain is one of them.

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Even at 26 weeks, a preterm infant is a tiny creature requiring intensive care for each gram of nourishment and every shallow breath. Breathing tubes and feeding tubes are needed, and often other special equipment as well. By contrast, after 37 weeks or more, 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. 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).

In the final three months of prenatal life, the lungs begin to expand and contract, and breathing muscles are exercised as the fetus swallows (into the lungs, not the stomach) and spits out amniotic fluid. The valves of the heart go through a final maturation, as do the arteries and veins throughout the body.

In those final prenatal months, the brain not only develops more neurons, but also connects them to each other, destroying neurons that are not functioning. As the body matures, the skin of the skull thickens. That prevents “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 an average of about 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) has formed 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 non-humans.

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. If she is up at dawn, her newborn infant is likely to be wide-awake then as well.

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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. Other voices are also heard, and many men begin to relate to their future child by stroking the mother’s abdomen and talking to the fetus, thrilling to feel the kick.

If the mother is fearful or anxious, the fetal heart beats faster and body movements increase, and later, the infant is likely to startle at noises and unexpected sights. Maternal stress in pregnancy affects the fetus, infant, and child in many ways—although scientists do not agree on exactly how stress should be measured and what all the effects of prenatal anxiety are (Graignic-Philippe et al., 2014). Fathers can mitigate stress, and many do, not only with reassuring words but also with massage, provisioning, and so on.

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