Now that we understand the starting point, let’s chart prenatal development, tracing how the microscopic, fertilized ovum divides millions of times and differentiates into a living child. This miraculous transformation takes place in three stages.

The first approximately two weeks after fertilization—when the cell mass has not fully attached to the uterine wall—is called the germinal stage (see Figure 2.4). Within 36 hours, the fertilized ovum, now a single cell called the zygote, makes its first cell division. Then the tiny cluster of cells divides every 12 to 15 hours as it wends its way down the fallopian tube. When the cells enter the uterine cavity, they differentiate into layers—some destined to form the pregnancy support structures, others the child-to-be. Now called a blastocyst, this ball of roughly 100 cells faces the challenge called implantation—embedding into the uterine wall.

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The blastocyst seeks a landing site on the upper uterus. Its outer layer develops projections and burrows in. From this landing zone, blood vessels proliferate to form the placenta, the lifeline that passes nutrients from the mother to the developing baby. Then, the next stage of prenatal development begins: the all-important embryonic phase.

Although the embryonic stage lasts roughly only six weeks, it is the most fast-paced period of development. During this time, all the major organs are constructed. By the end of this stage, what began as a clump of cells looks like a recognizable human being!

One early task is to construct the conduit responsible for all development. After the baby hooks up to the maternal bloodstream—which will nourish its growth—nutrients must reach each rapidly differentiating cell. So by the third week after fertilization, the circulatory system (our body’s transport system) forms, and its pump, the heart, starts to beat.

At the same time, the rudiments of the nervous system appear. Between 20 and 24 days after fertilization, an indentation forms along the back of the embryo and closes up to form the neural tube (see Figure 2.5). The upper part of this cylinder becomes the brain. Its lower part forms the spinal cord. Although it is possible to “grow” new brain cells throughout life, almost all of those remarkable branching structures, called neurons, which cause us to think, respond, and process information, originated in neural tube cells formed during our first months in the womb.

Meanwhile, the body is developing at an astounding rate. At day 26, arm buds form; by day 28, leg swellings erupt. At day 37, rudimentary feet start to develop. By day 41, elbows, wrist curves, and the precursors of fingers can be seen. Several days later, raylike structures that will become toes emerge. By about week 8, the internal organs are in place. What started out looking like a curved stalk, then an outer-space alien, now appears like a human being.

In scanning the photographs of the developing embryo below, can you spell out three guiding principles related to the sequence of development I just described?

Keep these principles in mind. As you will see in the next chapters, the same patterns apply to growth and motor skills after the baby leaves the womb.

During the embryonic stage, body structures literally sprout. In the fetal stage, development occurs at a more leisurely pace. From the eyebrows, fingernails, and hair follicles that develop from weeks 9 to 12, to the cushion of fat that accumulates during the final weeks, it takes seven months to transform the embryo into a resilient baby ready to embrace life.

Why does our species need this long refining period? One reason is to allow ample time for that masterpiece organ—the human brain—to form. Let’s now look at this process of making a brain.

During the late embryonic stage, a mass of cells accumulates within the neural tube that will eventually produce the more than 100 billion neurons composing our brain (Stiles & Jernigan, 2010). From this zone, the neurons migrate to a region just under the top of the differentiating tube (see Figure 2.6). When the cells assemble in their “staging area,” by the middle of the fetal period, they lengthen, develop branches, and interlink. This interconnecting process—responsible for every human thought and action—continues until almost our final day of life.

Figure 2.7 shows the mushrooming brain. Notice that the brain almost doubles in size from month 4 to month 7. By now, the brain has the wrinkled structure of an adult.

This massive growth has a profound effect. At around month 6, the fetus can hear (Crade & Lovett, 1988). By month 7, the fetus is probably able to see (Del Giudice, 2011). And by this time, with high-quality medical care, a few babies can survive. Today, the age of viability, or earliest date at which we can be born and possibly live, has dropped to 22 to 23 weeks—almost halving the 38 weeks the fetus normally spends in the womb. By week 25, in affluent nations, the odds of survival are more than fifty-fifty (Lawn and others, 2011).

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However, it is vitally important that the fetus stay in the uterus as long as possible. As I will describe later, being born too early (and too small) can make a lifelong impact in health.

Figure 2.8 shows the fetus during the final month of pregnancy, when its prenatal nest is cramped and birth looms on the horizon. Notice the baby’s support structures: the placenta, projecting from the uterine wall, which supplies nutrients from the mother to the fetus; the umbilical cord, protruding from what will be the baby’s bellybutton, the conduit through which nutrients flow; the amniotic sac, the fluid-filled chamber within which the baby floats. This encasing membrane provides insulation from infection and harm.

At this stage, parents may be running around, buying the crib or shopping for baby clothes. Middle-class women may be marveling at the items their precious son or daughter “must have”: a pacifier, a receiving blanket, a bassinet . . . and what else! What is happening during all nine months from the mother’s—and father’s—point of view?

Question 2.5

germinal; embryonic; fetal. Organs are formed during the embryonic stage.

Question 2.6

From the neural tube, a mass of cells differentiates during the late embryonic phase. During the next few months, the cells ascend to the top of the neural tube, completing their migration by week 25. In the final months of pregnancy, the neurons elongate and begin to assume their mature structure.

Question 2.7

(a) mass to specific (b) cephalocaudal (c) proximodistal

Question 2.8

Around 22-23 weeks