19.2: Pregnancy

The Placenta

The woman and her fetus don’t share a common blood supply. Instead, substances move between them through the intertwining of maternal and fetal blood vessels in an organ called the placenta (see Fig. 19-1). The placenta develops within the uterus during pregnancy. It connects to the fetus through the umbilical cord, and is discharged shortly after birth (the “afterbirth”). The fetus receives nourishment and discards waste products through the placenta.

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For simplicity, “fetus” is used to describe all stages of prenatal development. The technically correct terms are: zygote, for the first 2 weeks; embryo, for weeks 3 to 8; and fetus, from week 8 until birth.

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Many medications, including aspirin, can cross the placenta. So can caffeine, alcohol, cocaine, viruses, and substances absorbed into the bloodstream from cigarette smoke. Smoking during pregnancy appears to hinder fetal growth (women who smoke give birth to smaller babies), and may have other adverse effects as well.

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The placenta begins to form when the fertilized egg implants itself into the lining of the uterus.

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Although the precise effects (if any) of many substances passed to the fetus aren’t known, it goes without saying that one should exercise caution. Caffeine, for example, appears to be safe in moderate amounts. To be extra-cautious, however, it’s suggested that pregnant women restrict their daily caffeine intake to 300 mg, e.g., the amount in about 16 ounces (two cups) of coffee.¹

Fetal Alcohol Effects

Alcohol passes freely through the placenta, so the fetus’s blood-alcohol level is the same as the mother’s. Exposure to alcohol can affect the fetus’s developing brain, and is thought to be a common cause of intellectual disability.²

Fetal alcohol syndrome is the full-fledged syndrome, characterized by intellectual disability, a small head, growth deficiency, abnormal facial features (e.g., eye-openings that aren’t as wide as normal), and other conditions. Alcohol exposure during pregnancy can have milder effects. Fetal Alcohol Spectrum Disorder is the term used to describe the range of effects of prenatal alcohol exposure.

There has been considerable debate as to whether moderate or low doses of alcohol, especially late in pregnancy, damage the fetus. The controversy exists because of a lack of solid evidence of damage. Unlike deformities of the face or heart, subtle damage to the brain isn’t easily measured. Even behavior and learning disabilities are very difficult to assess, since it’s nearly impossible to distinguish alcohol effects from those of other environmental or genetic factors.

People sometimes think that if a child doesn’t have the characteristically abnormal facial features of fetal alcohol syndrome, the brain hasn’t been affected. But these facial abnormalities are formed only during the first three months of pregnancy, whereas the formation of the brain continues throughout fetal life and into early infancy.

Figure 19-1: Substances move between mother and fetus through the placenta.

Illogical behavior is common among alcohol-affected children. Those affected literally act as if they aren’t properly “connected,” perhaps because alcohol interfered with the development of pathways and connections in the brain. Animal studies show that fetal alcohol exposure can cause such effects, but it would be difficult, if not impossible, to pinpoint their causes in human studies.

Given the uncertainties, the safest choice for women who are pregnant or are trying to conceive is to avoid alcohol altogether.

Nutrient Needs in Pregnancy

Most women believe that special concern for their diets becomes necessary as soon as they become pregnant. In fact, such concern, and appropriate dietary changes ought to start even earlier. Often, a woman does not realize she is pregnant until she is already a month or two along. Very little growth in size occurs during the first two months, but there are crucial developmental changes. Also, the development of a healthy placenta during the first month of pregnancy is in large part dependent on the nutritional status of the mother when she conceives. A healthy placenta is needed for optimal growth and development of the fetus.

If the mother has eaten a nutritious diet beforehand, she need make little change during her first few months of pregnancy. The obstetrician uses these first few months to correct nutritional problems—such as iron-deficiency anemia—and to see to it that the mother-to-be has adequate stores for the last two-thirds of pregnancy.

A good diet is an important part of prenatal care. Except for iron, the dietary recommendations for pregnancy can easily be met by a sound diet. The RDA for iron during pregnancy is 27 mg—much more than before pregnancy (15-18 mg)—an amount much higher than that found in a regular diet. The reason for such a high iron RDA is that the majority of women come into pregnancy with low iron stores.

Most obstetricians, unsure about the adequacy of their patients’ diets, prescribe a prenatal supplement that includes (in addition to iron) vitamins and other minerals.

Folate (folic acid) deficiency in early pregnancy has been associated with neural tube defects. The neural tube (an embryonic structure that develops into the brain and spinal cord) is formed during early pregnancy. Folate deficiency—and neural tube defects—is now less common, because the addition of folate to refined grains (e.g., white flour) has been required since 1998. As an extra precaution, it’s recommended that all women capable of becoming pregnant take 400 mcg (100% DV) of folate in a multivitamin pill or in a fortified food product, e.g., a fortified breakfast cereal.

The Need for a More Nutrient-Dense Diet: “Eating for two” does not justify eating with abandon. Although the caloric cost of pregnancy is about 70,000 calories, this is an average of only 260 calories/day over the nine months. The calorie cost isn’t distributed evenly over the nine months. The recommended calories doesn’t increase in the 1st trimester, is an added 340 calories/day in the 2nd trimester, and an added 452 calories in the 3rd trimester.³ For a woman who requires 2,000 calories per day when not pregnant, an added 452 calories is only a 23% increase.

Figure 19-2: Increases in the RDA for certain nutrients during pregnancy

The need for many nutrients increases much more. There’s a big increase in the folate RDA (from 400 to 600 mcg) and as mentioned earlier, there’s a big increase in the iron RDA (from 15-18 to 27 mg). The RDA for some other nutrients are also increased (see Fig. 19-2). Because the general increase in RDA for nutrients is more than the increase in calories, a pregnant woman must eat a more nutrient-dense diet.

For women with poor diets, prenatal supplements provide some help. But taking supplements often gives a false sense of security—that the supplement insures adequate nutrition. Supplements don’t generally contain protein or fiber or very much calcium, and there may be other missing nutrients. A well-balanced diet, rather than relying on supplements, provides the best safeguard.

Also, excessive amounts of some nutrients can be harmful. Large doses of vitamin A, for example, can cause fetal malformations. One shouldn’t be haphazard in taking dietary supplements, especially during pregnancy.

For meeting nutritional needs, there are dietary guidelines for various life stages, including pregnancy myplate.gov/life-stages Again, because the increase in the recommended amounts of nutrients are larger than the increase in calorie need, a pregnant woman should make lower calorie choices (e.g., reduced-fat milk in the milk group)—and be more physically active.

The Vegan Mother: It’s harder to meet the nutrient needs of pregnancy on a vegan (plant foods only) diet. The most obvious challenge is vitamin B₁₂, which isn’t found in plant foods (see Chap. 14). It must be provided in plant foods fortified with B₁₂ (e.g., B₁₂-fortified soy milk) or as a prenatal supplement. B₁₂ is essential for proper development of the fetal nervous system.

It’s also a challenge for pregnant women on a vegan diet to get enough calcium, iron, and zinc. Part of the reason is that minerals from plant foods aren’t as well absorbed as they are from animal foods. Extra-special care in planning a vegan diet is important to assure adequate nutrition during pregnancy and lactation. Women who are vegans may want to consult a Registered Dietitian (RD) or Registered Dietitian Nutritionist (RDN) for diet counseling.

Weight Gain During Pregnancy

For a woman of normal weight, a gain of about 25-35 pounds is recommended during pregnancy, with most of the gain taking place during the last 6 months (see Fig. 19-3). Underweight women should gain a bit more, and overweight women a bit less. In no case should a woman—overweight or not—try to lose weight during pregnancy. This can have adverse effects on the fetus. Ideally, underweight and overweight women should attain a normal weight before becoming pregnant. For more precise information, go to: myplate.gov/node/5390

Birth Weight—A Predictor of Health for the Newborn: A woman’s pre-pregnancy weight and her weight gain during pregnancy are often the determining factors in the birth weight of her child. The birth weight, in turn, is a major predictor of health and survival of the newborn. Full-term babies weighing 7 to 8 pounds at birth have the greatest chance of infant survival and good health.

A full-term baby weighing 5 1/2 pounds or less at birth is at particularly high risk of developing health problems or not surviving the first year of life. In the U.S., such low-birth-weight babies are more common among African Americans. It is also more common among lower socioeconomic groups.

Figure 19-3: Components of weight gain during pregnancy.

Although several factors can contribute to low birth weight (e.g., smoking during pregnancy), a mother’s health and nutrition during and prior to pregnancy have a major impact. Unmarried young teenagers from lower socioeconomic groups are at greatest risk of delivering low-birth-weight infants. These young teens may be still growing themselves (increasing their nutrient needs even more), and receive little prenatal care.

The Nursing Mother

For the nursing mother, there are physiological advantages of breastfeeding. Among the poor in developing countries, a prominent advantage is that lactation (milk production) temporarily reduces a woman’s fertility, lessening her chances of becoming pregnant. Where food is scarce, a delay in another pregnancy can make a tremendous difference in her own health, and can be a matter of life or death for her child. The early displacement of an infant at the breast by the birth of a sibling is often a precipitating cause of severe malnutrition and its accompanying ills.

Another advantage in these poor situations is that breast feeding is a safe and sanitary method of feeding infants. Bottle feeding may not be, depending on local conditions (e.g., availability of clean water) and the way in which bottles are handled.

In the well-fed U.S. population, a prominent advantage for the mother is that breastfeeding helps her return to her pre-pregnancy weight. She “exports” calories in her breast milk, and also uses energy in producing the milk. The added calorie expenditure of a nursing mother increases in proportion to the amount of milk produced.

The estimated added calorie recommendation is 330 calories/day during the 1st six months. The actual calorie need is about 500 calories per day. The 170-calorie difference (500 minus 330) is expected to come from fat normally stored during pregnancy in anticipation of the added calorie needs of lactation.³ This extra fat storage amounts to about 8 pounds,4 enough to provide those 170 calories per day over a 6-month period of lactation.⁵ (Conversely, one can see that if a woman doesn’t breast-feed, she has to eat less and/or exercise more to lose the added fat stores.)

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Kwashiorkor—the severe and often fatal disease of protein-deficiency common in developing countries—is a local term in Gold Coast, Africa for displaced child.

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An interesting way to look at the added calorie allotments for pregnancy and lactation is this: For the last six months of pregnancy, a woman needs, on average, an extra 400 calories per day, and is expected to gain weight. For the first six months of breastfeeding, a woman needs an extra 500 calories per day, and is expected to lose weight (with 170 of those 500 calories coming from fat she stored during pregnancy). For some women, this is reason enough to breast-feed.

Just as there’s a need for a more nutrient-dense diet during pregnancy, so it is during lactation (see Fig. 19-4). However, the extra calorie allotment for lactation is a bit more than for pregnancy, whereas the increases in nutrient requirements are the same or a bit less.

Figure 19-4: Increases in the RDA for certain nutrients during lactation.

Almost all women are physically able to breastfeed. Breast size isn’t a factor. The American Academy of Pediatrics recommends, “exclusive breastfeeding for about 6 months, followed by continued breastfeeding as complementary foods are introduced, with continuation of breastfeeding for 1 year or longer as mutually desired by mother and infant.”⁵