Does that mean exercise should be a part of human pregnancies? Evidence suggests the answer is yes. The first benefit is a practical one, having to do with labor. Many women report that giving birth is both the most exhilarating experience of their lives and the most painful. But women who exercise regularly have a much easier time giving birth than obese women. For fit women, the second stage of labor—that painful phase where you have to do a lot of pushing—lasts an average of 27 minutes. Physically unfit women had to push for almost an hour, some far longer. Not surprisingly, fit women perceived this stage as being far less painful. And, because the pushing phase was so much shorter, their babies were less likely to experience brain damage from oxygen deprivation. If you are afraid of labor, you owe it yourself to become as fit as possible going into it. And the reasons are argued purely from the Serengeti.

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What is the proper balance? Four words: moderate, regular aerobic exercise. For most women, that means keeping your heart rate below 70 percent of its maximal rate (which is 220 beats per minute minus your age), then slowing things down as the due date approaches. But you should exercise. As long as you don’t have obstetric or other medical complications, the American College of Obstetricians recommends 30 minutes or more of moderate exercise per day.

Researchers have isolated three toxic types. Their common characteristic: that you feel out of control over the bad stuff coming at you. As stress moves from moderate to severe, and from acute to chronic, this loss of control turns catastrophic and begins to affect baby. Here are the bad types of stress:

In fact, some evolutionary biologists believe this is why morning sickness still persists in human pregnancies. Morning sickness, which can last the entire day (and, for some women, the entire pregnancy), makes a woman stick to a bland, boring diet—if she eats much at all. This avoidance strategy would have kept our maternal ancestors away from the natural toxins in exotic or spoiled foods in the wild, unregulated menu of the Pleistocene diet. The accompanying fatigue would keep women from engaging in physical activity risky enough to harm the baby. Researchers now think it could make the baby smarter, too.

Nonetheless, our early touch experiences determine the extent of possible tactile sensitivity. They also play a surprisingly potent role in the overall quality of brain development. We have already seen in Chapter 2 how rats raised in a highly enriched environment develop a thicker cerebral cortex and are actually cleverer than rats raised in a standard laboratory environment. A good share of this enriching experience involves tactile sensation. When young rats are provided with new toys, they excitedly paw, nuzzle, and climb atop them, increasing the electrical activity and ultimately the size of their somatosensory cortexes. If the same toys are left in the cage for several days, the rats grow bored with them, and their cortexes begin to shrink back in size. But if the toys are changed at least twice a week, the increases in cortical size persist.

It is almost frightening, as a parent (though not as a toy manufacturer!). to contemplate the implications of these experiments. Touch experience is essential not only for the development of touch sensitivity but for general cognitive development as well. Fortunately, toys are not the only source of touch stimulation that can elicit these changes. Comparable effects on rats' brains and psychological performance occur when pups receive extra grooming by their mothers or handling by experimenters during the early weeks of life. So we don't necessarily have to break the bank on toys to provide young children with adequate stimulation. Anything that increases a baby's variety of touch stimulation is likely to enhance many aspects of brain and mental development.

Of its several advantages, "birtn stress" has been found to be especially beneficial for a newborn's breathing. Compared with babies born by C'Section, vaginally delivered babies are quicker to take their first breaths; their blood oxygen levels rise more rapidly after birth; and they are less likely to suffer any of a number of respiratory problems in the first few hours of life. Even among babies delivered by C-section, those who undergo several hours of labor before delivery do much better than those delivered prior to the onset of labor, although not as well as vaginally delivered babies. Higher catecholamine levels explain much of the respiratory advantage of "stressed" babies, because these hormones are known to help absorb some of the excess liquid in the lungs at birth and to promote the release of lung surfactant detergent-like molecules that are necessary for gas exchange through the lung's tiny grapelike air cells, or alveoli. Other stress hormones, such as cortisol, probably also contribute to this last-minute lung maturation. Finally, vaginal delivery further aids the onset of breathing in a purely mechanical way: by helping squeeze some of this extra liquid out of the lungs as the baby's chest is compressed during passage through the birth canal.

Higher catecholamines also benefit vaginally delivered babies in other ways. Because catecholamines speed up metabolic rate, vaginally delivered babies are better able to maintain their body temperature, and they have larger reserves of glucose and other energy sources than C-section babies. They are also better adapted neurologically to life outside the womb, judging by their higher scores on tests of reflexes, muscle tone, and sensory responses during the first two days of life. Considering all these benefits of labor stress for the baby, some obstetricians now recommend that women planning to deliver by C-section first undergo at least the early stages of labor before surgery.

Of all the advantages of labor, some of the most intriguing are those that affect the baby's nervous system. There is some evidence that contractions even of the prelabor Braxton-Hicks type, promote brain development in sheep. Perhaps the additional touch and movement stimulation provided by contractions helps refine synaptic connections or promotes myelination during late gestation. Then, once true labor and delivery ensue, a baby's high catecholamine levels potently stimulate the nervous system. In adults, a large surge of adrenaline is highly arousing and can lead to a feeling of well-being. Catecholamines appear to have the same effect on newborns, who are more alert during the first two hours of life than for many days thereafter.

There are two main reasons for concern about exercising during pregnancy. One is that it may reduce the baby's oxygen supply, since exercise, like other sources of stress, reduces blood flow to the uterus. Another risk is overheating. As we have already seen, fetal development is highly sensitive to temperature, and elevations of more than 2^0 C (or above 1020F) can increase the risk of miscarriage and affect the formation of the brain and eyes.

Despite these theoretical concerns, there is little evidence that mothers who exercise or are physically very active have any particular problems with their pregnancies. Most studies have found no difference in prematurity or Apgar scores (measures of newborn health taken one and five minutes after birth) between babies born to mothers who exercise and those who are more sedentary. With regard to birth weight, there are conflicting reports about the effects of exercise; some studies have found that women who exercise have significantly smaller babies, but several recent studies refute these findings, and one actually found that the more women exercised, the larger their babies tended to be. The key factor here appears to be the amount of weight the mother gains. If exercise prevents the mother from gaining adequately. she is likelier to give birth to a low-weight baby, but women who exercise and gain sufficient weight do not appear to compromise their baby's brain and bodily development.

Offsetting its potential harm are the numerous benefits of exercise, many of which can be traced to the fact that it elevates a mother's levels of betaendorphin—-a morphine-like substance produced by the body that blocks the transmission of painful stimuli to the brain. In addition, exercise actually lowers the level of another stress hormone, Cortisol, in pregnant women. These hormonal changes explain why exercise often counteracts the emotional impact of other sources of stress. Exercise generally increases a woman's sense of well-being, and based on what we know about anxiety and stress, this is likely to have a positive influence on the fetus.

The best-documented benefit of exercise comes in labor and delivery. Women who exercise regularly fare much better during childbirth compared with women who do not. They perceive it to be less painful, and indeed it may be; one study found that women who exercise spend just twenty-seven minutes in the second stage of labor—pushing—compared with fifty-nine minutes for women who did not exercise during pregnancy. Shorter labor is generally beneficial to the baby, since it reduces the risk of complications. including oxygen deprivation of the brain.

Doctors have traditionally been rather conservative about exercise during pregnancy, but current evidence indicates that it is safe for most women, especially those who were already physically fit before conceiving. Exercise should be kept to a "moderate" level, meaning that it does not elevate the woman's heart rate above 70 percent of its maximum rate (220 beats per minute minus one's age in years)—for example, 133 beats per minute in a thirty-year-old. Because there is evidence that a woman's oxygen reserves are lower in the third trimester, it is a good idea to scale down exercise, particularly weight-bearing types, toward the end of pregnancy, as most women are lined to do anyway. Other situations to avoid include: (1) exercising at high altitudes (more than 10,000 feet), because the placenta is already having to compensate for lower oxygen levels; (2) exercising in hot weather. because of the risk of overheating the fetus; and (3) scuba and snorkel diving, because of the potential risk of accumulating excess nitrogen and other gases in fetal tissues. But other water immersion sports, like swimming and "aqua-jogging," are among the best forms of exercise for pregnant women. because the water helps dissipate excess heat from the mother's body.

In the case of a mother's more general nutritional status—her tota caloric intake—the brain is actually less sensitive during the first three to four months of gestation. In spite of its massive developmental changes, the fetus grows surprisingly little in size during this period, so its growth is not very dependent on the mother's diet. (This is probably no accident, since women are often unable to consume many calories because of first-trimester nausea.) Beginning around midway through gestation, however, and continuing until about two years after birth, the brain's growth is highly sensitive to the quantity and quality of nutrition it receives. This sensitive period coincides with the great spurt in synapse development, dendritic growth, and myelination, which together wire up the brain and also greatly increase its total weight. The quality o{ nutrition during this period has a profound impact on a child's future cognitive, emotional, and neurological functions.

Because this sensitive period begins before birth, it means that a mother's diet can shape her baby's brain development. And because it continues throughout infancy and toddlerhood, it means that special attention must be paid to a child's diet during these first two years. Nutritional deficits can be very specific, such as insufficient iodine, iron, or vitamin B12 intake, each of which can permanently alter brain and cognitive development if it continues for any substantial portion of the sensitive period. It is more common. however, for young children to suffer from a generalized nutritional deficiency—too few calories during gestation and early life—that can permanently compromise their brain development. Insufficient nutrition threatens the brain if it occurs at any time during the sensitive period, but is more devastating the earlier in this period it occurs and the longer it lasts, and when the lack of calories is compounded by inadequate protein intake.

The effects of malnutrition have been thoroughly studied in experimental animals, where we have achieved a fairly detailed understanding of the timing and type of nutrients needed for optimal brain development. Unfortunately, plenty of data are also available for human populations. A large proportion of children in the world are undernourished because of famine, poverty, war, and other natural or man-made disasters. It is through studies of such children that we have learned the ways in which inadequate early nutrition can permanently impair brain function. Children who were undemourished as fetuses or infants tend to score lower on IQ tests, perform more poorly in school, have slower language development, exhibit more behavioral problems, and even have difficulties with sensory Integration and fine motor skills, compared with children from the same culture who were adequately nourished. The earlier the malnourishment begins (starting with midpregnancy) and the longer it lasts, the greater will be the resulting problems and the less likely they can be overcome later on. By comparison, adults who undergo even the most extreme starvation do not suffer any intellectual impairment. Thus the brain has a special sensitive period for nutrition in infancy corresponding to the phase of massive synapse growth and axon myelination, both of which require considerable metabolic energy.

Babies of malnourished mothers are small at birth, with correspondingly smaller head sizes than babies well nourished in the womb. Within the normal range, birth weight and head size are only modestly related to later intelligence. But babies in the lowest tenth percentile, whose birth weight is less than four and a half pounds, do have a higher incidence of neurological impairment and mental deficits than larger infants. And malnourished babies are very likely to be in this smallest group.

Birth weight, unlike many traits, is influenced much more by a mother's nutrition than by heredity. Optimally, a pregnant woman should gain about 20 percent of her ideal prepregnancy weight (for example, twenty-six pounds for a 130-pound woman). Bigger is generally better, but there is a limit. (See Figure 17.3.) Babies who are very large at birth are likelier to cause a difficult delivery, and the brain is the organ most vulnerable to damage during a complicated birth. For optimal development a woman needs to consume about 300 extra calories per day during pregnancy, and 500 to 600 extra calories during lactation. It is recommended that many of these additional calories come from protein, which is especially important for brain development; women are advised to consume an extra lo to 12 grams of protein per day during pregnancy and 12 to 15 grams during lactation.

As we learn more about maternal hormones and their influence on the developing brain, scientists are beginning to propose actual biological mechanisms for the kind of folk prophecies that have been around for ages. One recent study, for instance, suggests that a child's shyness is determined, in part, by maternal hormone fluctuations during gestation. Researchers who interviewed several thousand preschoolers in both the United States and New Zealand noted a significant relationship between the incidence of extreme shyness or inhibition (children who seem particularly fearful, anxious, or withdrawn in the presence of a stranger) and the amount of daylight their mothers were exposed to at midpregnancy. Thus, in the United States, only 12 percent of children born in October-November-December were rated as highly inhibited, compared to nearly i8 percent of those born in April-May-June. In New Zealand, where daylight hours are reversed, children showed the opposite pattern, with more shy children born in October-November-December than in April-May-June. Because the production of certain hormones, like melatonin, is known to fluctuate with the amount of daylight in each season, the researchers propose that such substances may subtly alter brain development during a critical period at midgestation, when massive numbers of neurons are migrating to form the basic architecture of the cerebral cortex. (It is also possible that other seasonal differences, like changes in women's diets, physical activity, or exposure to colds and flu. mediate this relationship.)

Caffeine crosses the placenta and may even concentrate in the fetal circulation. Concern about its effect on fetal development stems from animal studies, where it has been found to be teratogenic when fed to pregnant rats in high doses; a dose equivalent to 150 cups of strong coffee per day causes malformations in rodents such as missing limbs and digits. However, caffeine does not appear to be a teratogen in h humans. The average pregnant woman is estimated to consume 144 milligrams per day of caffeine, which presents no danger to a developing fetus. Even women who consume fairly large amounts of caffeine (more than 400 milligrams per day) during pregnancy do not increase their risk of having a baby with a congenital malformation. And despite old wives' warnings, prenatal exposure to caffeine does not stunt fetal growth; nor, according to one study of seven-year-olds whose mothers drank caffeinated beverages during pregnancy, does it have any effect on a child's later IQ.