Welcome to Your Child's Brain: How the Mind Grows From Conception to College (6 page)

Similar studies have yielded comparable results. One found that children whose mothers experienced severe stress from a major ice storm while pregnant had lower intelligence quotient (IQ) scores and language ability at age five. The risk of schizophrenia is higher in children whose mothers were in the first trimester of pregnancy when a close relative died or was diagnosed with a serious illness. Children whose mothers experienced an earthquake during pregnancy were more likely to be diagnosed with depression or to be born with a cleft palate. It’s not yet clear whether moderate stress, such as dealing with an annoying boss, might cause similar problems, but as the research is ongoing, it’s best to keep it simple: it’s probably a good idea to take time to relax and be kind to yourself during pregnancy as much as you can.

Once axons are close to their destination, they begin to make contacts with nearby cells, initiating the chemical conversation that leads to the formation of synapses. This process begins in the spinal cord five weeks after fertilization, and it is not complete until years after birth in some brain areas. Axons initially form a lot of extra synapses with targets that are only roughly appropriate. Only some of these synapses survive in the long term. Synapses that are more successful at activating their target cells are more likely to be retained. This competition for synaptic survival provides a way of fine-tuning the brain’s function to match each child’s individual circumstances, whether that means adapting the responses of vision neurons to the distance between each child’s eyes or tuning the auditory cortex to respond most easily to the sounds of each child’s native language. To a lesser extent, this process will continue throughout life, as a mechanism of learning and memory (see
chapter 21
).

The process of eliminating unnecessary components is a major theme of early development. The adult brain contains about 100 billion neurons and many more glia. However, the young brain produces even more cells than that and then reduces their number through planned cell death. In some brain regions, planned death kills as many as four out of every five cells born. These events are called
regressive
by neuroscientists, and they are essential for normal development.

Why does the nervous system take such a wasteful approach? It seems to be a way of matching the size of the incoming population of axons to the number of neurons in the target region. Cell death occurs after the axons have reached their target and formed synapses. The target neurons produce a protein, necessary for cell survival, which is taken up at synapses and transported back along the axon to the cell body of the input neuron. Cells that have failed to form enough connections with the target do not get enough of the survival substance, so they die. This type of cell death is an active process, resulting from a biochemical death pathway within the cell. The best-known survival protein (or
neurotrophin
) is nerve growth factor, which controls the survival of neurons involved in the sense of touch and the fight-or-flight reflex in the peripheral nervous system. Other factors also influence cell survival, including incoming synaptic activity and sex hormones, which control cell death in brain regions that differ between males and females.

Even after all the cellular elements of the brain are in place, much construction work remains to be done. Newborn neurons look very simple compared with mature neurons. Toward the end of gestation and especially in the first two years of life, dendrites form additional branches, becoming more and more complex to accommodate the many new synapses that are added during this period. Synapse elimination begins in the first year of life and continues through early adolescence, forming one of the basic mechanisms by which experience helps to shape the brain (see
chapter 5
).

The final step in axon maturation is myelination, the formation of the glial insulation that allows spikes to move quickly down the axon. It’s as if the brain’s
electrical system were installed with bare wires, and then the insulation got added afterward. This process begins just before birth in the brain (earlier in the spinal cord) and continues well into adult life (see
chapter 9
).

Considering the enormous amount of construction involved, it’s no surprise that the growing baby requires energy. Indeed, one of the biggest threats to a developing fetus is maternal malnutrition, whether caused by famine, poverty, or dieting. A particularly critical time is the second and third trimesters, when brain size is increasing rapidly. An unexpectedly low birth weight (compared with the baby’s expected genetic growth potential) signals a higher risk of many problems later in life, including deficits in cognitive development and intelligence. Low birth weight and other problems, such as mental retardation and inflammation of the
retina
, are also associated with a variety of viral infections, including toxoplasmosis, rubella, and herpes simplex. In general, it is prudent to practice good hygiene late in pregnancy, and for that matter around young babies. High birth weight can be unhealthy as well. Customized growth curves do exist. Ask your obstetrician about them.

During pregnancy, environmental toxins can be a threat if they are ingested. For instance, cocaine use increases the risk of attention-deficit/hyperactivity disorder (ADHD). However, more severe effects on the brain result from two legal drugs, nicotine and alcohol. Low birth weight and a variety of brain development problems are linked to smoking, the nicotine patch, and heavy drinking. So-called crack babies, whose plight got a lot of press in the 1980s, turned out to be damaged mainly by their mothers’ malnutrition and concurrent use of other drugs.

In the
Mad Men
era, the sight of a pregnant woman with a drink in one hand and a cigarette in the other did not attract a second glance. Today, some U.S. states jail women for child abuse if they are caught taking cocaine while pregnant, but not for smoking or alcoholism. To put it mildly, this approach is not optimal for the baby’s health (see
Practical tip: Less stress, fewer problems
). Although prison time for lighting up while pregnant is not likely anytime soon, you can improve the health of your baby by addressing these habits early in pregnancy—or better yet beforehand.

PRACTICAL TIP: EAT FISH DURING PREGNANCY

Exposure to lead (from paint, water pipes, dishes, and even imported makeup) or mercury in utero or in childhood can decrease intelligence. These heavy metals are harmful to brain development. For years, women were told to limit their consumption of fish because it might contain mercury. But fish is also a major source of omega-3 fatty acids, which are crucial for neural development. Indeed, their absence during brain formation can lead to mental retardation. To settle this matter, we dug into the scientific literature to weigh the risks and benefits. Our verdict: fish is good. Several long-term studies now show that children whose mothers eat fish during pregnancy have better-functioning brains than children whose mothers avoid fish—especially if the mother chooses fish species that are low in mercury.

One group of researchers evaluated the eating habits of 11,875 women living in Bristol, United Kingdom, during the third trimester of pregnancy and then tested the resulting children on a variety of cognitive measures. Mothers who avoided seafood were more likely to have children with poor social behavior (at age seven) and low verbal IQ (at age eight) than mothers who ate at least three six-ounce (170-gram) servings per week. The more seafood a mother ate, the better her child’s brain functioned, which suggests that the effect was due to the fish itself and not to related characteristics such as household wealth. The benefit is small but well documented. (A statistical estimate of effect size is about 0.2 to 0.3; see
chapter 8
for a discussion of this measure.) No benefits were seen among mothers who ate fewer than two servings a week. Another study confirmed these findings and further showed that children of mothers who ate fish low in mercury during pregnancy had higher verbal intelligence than children whose mothers ate fish high in mercury, for the same fish consumption.

You may have heard that uncooked fish, especially wild Pacific salmon, can contain disease-causing parasites. Thorough freezing of the fish to kill the parasites, which is required of sushi consumed in the United States, minimizes this risk.

How do you know if your fish contains mercury? A good rule of thumb is the smaller the fish, the less mercury it is likely to contain. Top predators like swordfish and shark should be avoided because mercury and other contaminants get concentrated as they go up the food chain. Your health department may have information on the risks associated with your local fish. The most important point, though, is that the benefit of getting enough omega-3 fatty acids seems to outweigh the risk of mercury contamination for fetal brain development.

Another source of drug exposure during pregnancy is medical care. Pregnant mothers are advised to avoid a variety of over-the-counter drugs. In regard to brain growth, the third trimester is a time of vulnerability. Drugs can enter the placenta, and therefore the baby’s developing brain, and increase the likelihood of neurodevelopmental problems. One drug given at this stage is terbutaline, an activator of receptors for the neurotransmitter
epinephrine
, which is intended to prevent preterm labor—but is in fact ineffective at doing so. Steroids that emulate the stress response are given to improve lung development in babies at risk for preterm birth, but multiple courses of steroids can harm the developing brain. Even for drugs that are currently regarded as safe, the possibility of a risk exists late in pregnancy, when the baby’s brain is growing rapidly. Though in many cases there is no alternative, risks and demonstrated benefits for both mother and baby should be weighed carefully.

The news is not all bad for drugs, though. One of our favorites, caffeine, is harmless in moderate doses of no more than 300 milligrams a day (three regular cups of coffee—or a single Starbucks grande), as are artificial sweeteners and monosodium glutamate (better known as MSG). Although it is generally not recommended, there are some doctors who even approve of alcohol in small doses. So expectant mothers don’t need to give up all of their favorite habits. Indeed, a little less worry on this front might be a stress reducer.

An important threat to the baby occurs when the pregnancy cannot run its full course. A common cause of low birth weight is premature birth, which greatly increases the risk of neurodevelopmental disorders. One Norwegian study found that babies born during gestational weeks twenty-eight to thirty have a fourfold higher incidence of mental retardation, a sevenfold higher incidence of autism spectrum disorder (see
chapter 27
), and a forty-six-fold higher rate of cerebral palsy than full-term babies. By the age of eighteen, one in twelve of these children was classified as disabled—five times the normal rate. Premature babies born later in gestation have lower rates of disability, but even at thirty-seven weeks, the risks remain elevated over babies born at full term. A major contributor to preterm birth is carrying more than one baby. Couples using in vitro fertilization can reduce this risk by asking their doctors to transfer no more than one embryo per cycle.

One of the biggest threats to a developing fetus is maternal malnutrition, whether caused by famine, poverty, or dieting.

One unintended consequence of recent changes in medical practice is an increase in the frequency of premature birth. Preterm babies (less than thirty-seven weeks) now make up 12–13 percent of births in the U.S. This percentage rose steadily between 1981 and 2004, partly because their survival rates grew as medical care improved. Three quarters of preterm births occur late in gestation, between thirty-four and thirty-seven weeks. Among babies born in this time range, 20 percent end up with clinically significant behavior problems, and the risk of ADHD is 80 percent higher than in full-term babies.