You probably calculate your age by counting your birthdays. But the fact is that on the day you were born, you were already 9 months old. The prenatal stage of development ends with birth and begins 9 months earlier when about 200 million sperm make the journey from a woman’s vagina, through her uterus, and on to her fallopian tubes. That journey is a perilous one. Many of the sperm have defects that prevent them from swimming vigorously enough to make any progress, and others get stuck in the spermatazoidal equivalent of a traffic jam in which too many sperm are headed in the same direction at the same time. Of those that do manage to make their way through the uterus, many will take a wrong turn and end up in the fallopian tube that does not contain an egg. In fact, a mere 200 or so of the original 200 million sperm will manage to find the correct fallopian tube and get close enough to an egg to release digestive enzymes that erode the egg’s protective outer layer. The moment the first sperm manages to penetrate the egg’s coating, the egg will release a chemical that seals the coating and keeps all the other sperm from entering. After triumphing over 199 999 999 of its closest friends, this single successful sperm will shed its tail and fertilize the egg. About 12 hours later, the egg will merge with the nuclei of the sperm, and the prenatal development of a unique human being will begin.

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A zygote is a fertilized egg that contains chromosomes from both an egg and a sperm. From the first moment of its existence, a zygote has one thing in common with the person it will someday become: sex. Each human sperm and each human egg contain 23 chromosomes. One of these chromosomes (the twenty-third) comes in two varieties known as X and Y. An egg always has an X chromosome, but a sperm can have either an X or a Y chromosome. If the egg is fertilized by a sperm that has a Y chromosome, then the zygote is male (XY), and if it is fertilized by a sperm that has an X chromosome, then the zygote is female (XX).

The germinal stage is the 2-week period of prenatal development that begins at conception. During this stage, the one-celled zygote divides into two cells that then divide into four cells that then divide into eight cells, and so on. By the time an infant is born, its body contains trillions of cells, each of which came from the original zygote, and each of which contains exactly one set of 23 chromosomes from the sperm and one set of 23 chromosomes from the egg. During the germinal stage, the zygote migrates back down the fallopian tube and implants itself in the wall of the uterus. This too is a difficult journey, and about half of the zygotes do not complete it, either because they are defective or because they implant themselves in an inhospitable part of the uterus. Male zygotes are especially unlikely to complete this journey and no one understands why (though several comedians have suggested that it is because male zygotes are especially unwilling to stop and ask for directions).

If the zygote successfully implants itself in the uterine wall, it earns the right to be called an embryo and a new stage of development begins. The embryonic stage is a period of prenatal development that lasts from the second week until about the eighth week (see FIGURE 11.1). During this stage, the embryo continues to divide and its cells begin to differentiate. Merely 2.5 cm long, the embryo already has a beating heart and other body parts, such as arms and legs. Embryos that have XY chromosomes begin to produce a hormone called testosterone, which masculinizes their reproductive organs.

At about 9 weeks, the embryo gets a new name: fetus. The fetal stage is a period of prenatal development that lasts from the ninth week until birth. The fetus has a skeleton and muscles that make it capable of movement. It develops a layer of insulating fat beneath its skin, and its digestive and respiratory systems mature. The cells that will ultimately become the brain divide very quickly around the third and fourth week after conception, and this process is more or less complete by 24 weeks. During the fetal stage, brain cells begin to generate axons and dendrites (which permit communication with other brain cells). They also begin to undergo a process (described in the Neuroscience and Behaviour chapter) known as myelination, which is the formation of a fatty sheath around the axons of a neuron. Just as plastic sheathing insulates a wire, myelin insulates a brain cell and prevents the leakage of neural signals that travel along the axon. This process starts during the fetal stage but does not end for years; the myelination of the cortex, for example, continues into adulthood.

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Although the brain undergoes rapid and complex growth during the fetal period, at birth it is nowhere near its adult size. Whereas a newborn chimpanzee’s brain is nearly 60 percent of its adult size, a newborn human’s brain is only 25 percent of its adult size, which is to say that 75 percent of a human’s brain development occurs outside the womb. Why are humans born with such underdeveloped brains?

First, adult humans have huge heads. If a newborn human’s head were 60 percent of its adult size—like a newborn chimp’s head is—then that human would never be newborn because it could never pass through its mother’s birth canal. Second, one of our species’ greatest talents is its ability to adapt to a wide range of novel environments that differ in climate, social structure, and so on. Rather than arriving in the world with a fully developed brain that may or may not meet the requirements of its environment, human beings arrive with brains that do much of their developing within the very environments in which they ultimately must function. The fact that our underdeveloped brains are specifically shaped by the unique social and physical environment into which we are born is one of the main reasons why we are so adaptable.

The womb is an environment that has a powerful impact on development (Coe & Lubach, 2008; Glynn & Sandman, 2011; Wadhwa, Sandman, & Garite, 2001). For example, the placenta is the organ that physically links the bloodstreams of the mother and the embryo or fetus and permits the exchange of certain chemicals. That is why the foods a woman eats during pregnancy can affect her unborn child. The children of mothers who receive insufficient nutrition during pregnancy often have physical problems (Stein et al., 1975) and psychological problems, most notably an increased risk of schizophrenia and antisocial personality disorder (Neugebauer, Hoek, & Susser, 1999; Susser, Brown, & Matte, 1999). The foods a woman eats during pregnancy can also shape her child’s food preferences: Studies show that infants tend to like the foods and spices that their mothers ate while they were in utero (Mennella, Johnson, & Beauchamp, 1995).

But it is not just food that affects the fetus. Almost anything a woman eats, drinks, inhales, injects, sniffs, snorts, or rubs on her skin can pass through the placenta. Agents that impair prenatal development are called teratogens, which literally means “monster makers.” Teratogens include environmental poisons such as lead in the water, paint dust in the air, or mercury in fish, but the most common teratogens can be purchased at your local liquor or convenience store. Fetal alcohol spectrum disorder (FASD) is a developmental disorder that stems from heavy alcohol use by the mother during pregnancy. It is called a “spectrum disorder” because it manifests in different levels of severity that depend on many factors such as the amount the mother drinks, how often she drinks, and whether periods of heavy drinking coincided with critical periods of development in the embryo and fetus. Children with FASD have a variety of brain abnormalities and cognitive deficits (Carmichael Olson et al., 1997; Streissguth et al., 1999). The prevalence of FASD is estimated to be approximately 9.1 per 1000 live births, based on American data from Seattle, Washington (Sampson et al., 1994). However, prevalence varies dramatically across communities—some Canadian First Nations communities have much higher estimated prevalence (between 25 and 101 per 1000) (Square, 1997; Asante & Nelms-Maztke, 1985). Canada has a strong international reputation in the field of FASD research, and Canadian researchers are currently investigating the abnormal development of the brain in FASD, prevention of FASD in Aboriginal communities, and interventions that may help to prevent or reduce the impact of disabilities associated with prenatal alcohol exposure.

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Tobacco is the other common teratogen, and there is no debate about its effects. About 20 percent of American mothers admit to smoking during pregnancy (Substance Abuse and Mental Health Services Administration, 2005). The half-million infants to which they annually give birth are smaller (Horta et al., 1997) and more likely to have perceptual and attentional problems in childhood (Espy et al., 2011; Fried & Watkinson, 2000). Even second-hand smoke can lead to reduced birth weight and deficits in attention and learning (Makin, Fried, & Watkinson, 1991; Windham, Eaton, & Hopkins, 1999). The embryo is more vulnerable to teratogens than is the fetus, but structures such as the central nervous system remain vulnerable throughout the entire prenatal period. Approximately 10 percent of women in Canada smoke during pregnancy (Al-Sahab, Saqib, Hauser, & Tamim, 2010), and 11 percent are exposed to second-hand smoke during pregnancy (Crane et al., 2011). Both maternal smoking and second-hand smoke have been associated with a higher risk of stillbirth and infant death before the first birthday, as well as lower birth weight (birth weight is a general indicator of the health and maturity of the baby), and a smaller head circumference (Crane et al., 2011).

The prenatal environment is rich with chemicals, and it is also rich with information. Unlike an automobile, which operates only after it has been fully assembled, the human brain is operating while it is being built, and research shows that the developing fetus can sense stimulation and learn from it. Wombs are dark because only the brightest light can filter through the mother’s abdomen, but they are not quiet. The fetus can hear its mother’s heartbeat, the gastrointestinal sounds associated with her digestion, and her voice. How do we know? Newborns will suck a nipple more vigorously when they hear the sound of their mother’s voice than when they hear the voice of a female stranger (Querleu et al., 1984), demonstrating that they are more familiar with the former. Similarly, newborns whose mothers read aloud from The Cat in the Hat during their pregnancies react as though the story is familiar (DeCasper & Spence, 1986). Newborns who are presented with words from two languages prefer hearing their mother’s native language—unless their mother is bilingual, in which case they are just as happy to hear both languages (Byers-Heinlein, Burns, & Werker, 2010). What newborns hear even influences the sounds they make at birth: French newborns cry with a rising melody and German newborns with a falling melody, mimicking the cadence of their mother’s native tongue (Mampe et al., 2009). Clearly, the fetus is listening.

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