Nutrition During Pregnancy

Low or Underweight Preconception Body Mass Index

The 2009 IOM guidelines are based upon the World Health Organization (WHO) classifications to define underweight, normal weight, overweight, and obese patients ( Table 6.1 ). Underweight is classified as a BMI less than 18.5 kg/m ² . Based on the available data, there is strong support that women with low pregnancy BMI and low gestational weight gain have an increased risk for having small-for-gestational-age (SGA) infants, preterm birth, and perinatal mortality. In contrast, excessive gestational weight gain for women with a low pregravid BMI increases risk of large-for-gestational-age neonates and for increased maternal weight retention postpartum ( Fig. 6.2 ; also see Chapters 6 and 41 ).

Overweight and Obese Prepregnancy Body Mass Index

Approximately 60% of reproductive-age women are overweight (BMI >25 kg/m ² ), and of these, 50% are obese (BMI >30 kg/m ² ). Another 8% have severe obesity, with a BMI greater than 40 kg/m ² . Obesity poses a multitude of threats related to pregnancy: antepartum risks include spontaneous abortion, congenital anomalies, preterm birth, gestational diabetes, hypertension, and preeclampsia; during labor, risk of shoulder dystocia and cesarean delivery are higher; and postpartum risks include thromboembolic events, anemia, and incision-site infections (see Chapters 6 and 41 ).

The 2009 IOM guidelines recommend that overweight women with a singleton pregnancy gain a total of 15 to 25 lb during pregnancy (compared with 25 to 35 lb for normal-weight women). Obese women are advised to gain only 11 to 20 lb during the course of their pregnancy. These recommendations were based primarily on class I obesity because data are limited regarding weight gain recommendations for each class of obesity. As such, the IOM guidelines do not differentiate among class I obesity (BMI 30 to 34.9 kg/m ² ), class II obesity (BMI 35 to 39.9 kg/m ² ), and class III obesity (BMI ≥40 kg/m ² ). The 11 to 20 lb gestational weight gain for obese patients primarily represents the obligatory weight gain of pregnancy—approximately 12 to 14 lb of water, 2 lb of protein, and a variable amount of adipose tissue ( Table 6.2 ).

Some authors have recommended less gestational weight gain for obese class II and III women to decrease neonatal morbidity. However, other studies have noted an increased risk of SGA babies in overweight and obese women with inadequate gestational weight gain. Currently for obese pregnant women with less weight gain than recommended by the IOM guidelines but with an appropriately grown fetus, no evidence exists that encouraging increased weight gain will improve maternal or neonatal outcomes.

Multiple Gestations

For twin pregnancies, the IOM recommends a gestational weight gain of 37 to 54 lb for women of normal weight, 31 to 50 lb for overweight women, and 25 to 42 lb for obese women ( Table 6.3 ). There are insufficient data and thus no recommendations for triplet and higher-order gestations.

Adolescents

Approximately 17% of teenage girls ages 12 through 19 are obese in America, and these obese teens face the same obstetric risks that adult obese women face. Because 80% of teen pregnancies are unintended and therefore the window to discuss preconception nutrition is missed, the efforts to discuss appropriate gestational weight gain and adequate nutrition should be made during early prenatal care visits. Studies have shown that teens who are obese before conceiving are more likely to remain obese postpartum. The IOM recommendations state that adolescents should follow the adult BMI categories to guide their weight gain.

Other Groups

No specific recommendations exist for gestational weight gain in women of short stature, for specific racial or ethnic groups, or in those who smoke cigarettes. The IOM guidelines recognize each of these special groups may benefit from specific gestational weight gain guidelines, but the available evidence is insufficient to make recommendations.

Energy

Energy expenditure consists of four basic components: (1) resting metabolic rate (RMR); (2) the thermic effect of food (TEF), or diet-induced thermogenesis; (3) the thermic effect of energy (TEE); and (4) adaptive, or facultative, thermogenesis. RMR is the amount of energy, or calories, used at rest and accounts for approximately 60% of total energy expenditure in healthy people. TEF is the caloric cost of eating, digesting, absorbing, resynthesizing, and storing food, which accounts for 5% to 10% of total energy expenditure. TEE is quite variable and in sedentary individuals may account for only 15% to 20% of total energy expenditure. Adaptive thermogenesis, or facultative thermogenesis, refers to adaptations by an organism to adjust to environmental changes, such as overfeeding or underfeeding, as well as to alterations in ambient temperature. Adaptive thermogenesis accounts for no more that 10% of RMR and varies by individual ( Fig. 6.3 ).

The total maternal energy requirement for a full-term pregnancy is estimated at 80,000 kilocalories (kcal). This accounts for the increased metabolic activity of the maternal and fetal tissues as well as for the growth of the fetus and placenta. Maternal energy needs are increased to support the maternal cardiovascular, renal, and respiratory systems. Basal requirements can be determined based on maternal age, stature, activity level, preconception weight, BMI, and gestational weight gain goals. Daily caloric requirements have been estimated by the WHO by dividing the gross energy cost of pregnancy (80,000 kcal) by the approximate duration of pregnancy (250 days after the first month), producing an average additional 300 kcal/day for the entire pregnancy. During the first trimester, total energy expenditure does not change greatly and weight gain is minimal, assuming the woman began her pregnancy without depleted body reserves. Therefore additional energy intake is recommended primarily in the second and third trimesters. During the second trimester, a pregnant woman should consume an additional 340 kcal/day above the nonpregnant energy requirement, and in the third trimester, the additional caloric requirement is 452 kcal/day . However, a series of prospective studies conducted in the late 1980s and 1990s to assess energy expenditure in pregnancy showed that the energy cost of pregnancy is much lower than previously estimated.

The introduction of the doubly labeled water method has allowed researchers to estimate total energy expenditure in the free-living state. In well-nourished women, RMR usually begins to rise soon after conception and continues to rise until delivery. However, considerable interindividual variation is apparent. In healthy, well-nourished women, the average increases in RMR above prepregnancy values are 4.5%, 10.8%, and 24.0% for the first, second, and third trimesters, respectively.

Pregnancy induces changes in fuel utilization. Later in pregnancy, the basal or fasting contribution of carbohydrates to oxidative metabolism increases. Fetal growth and lactation are dependent on energy preferentially derived from carbohydrates.

Proteins

Additional protein is required during pregnancy for fetal, placental, and maternal tissue development. During the course of the pregnancy, an average of 925 g of protein are stored, which is required for the tissue development. Maternal protein synthesis increases to support expansion of the blood volume, uterus, and breast tissue. Fetal and placental proteins are synthesized from amino acids supplied by the mother. Protein recommendations are therefore increased from 46 g/day for an adult, nonpregnant woman to 71 g/day during pregnancy. This represents a change in protein recommendation from 0.8 g/kg per day for nonpregnant women to 1.1 g/kg per day during pregnancy.

Omega-3 Fatty Acids

Polyunsaturated fatty acids (PUFAs) are an essential component of neural tissue and are found in high concentrations in membrane phospholipids of the gray matter and the retina. These critically important fatty acids cannot be synthesized by the body and thus must be consumed in the diet as either linoleic or α-linolenic acid. After ingestion, α-linoleic acid is converted to its biologically active forms, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The amount of fatty acids that are transported to the fetus depends both on maternal dietary intake and placental function. With n-3 PUFA supplementation, randomized, placebo-controlled studies have shown a positive relationship between maternal intake and umbilical cord concentrations.

The best dietary sources for PUFAs are seafood—namely, oily fish such as salmon, sardines, and anchovies—as well as oils from some plants, such as flax seeds and walnuts. The Food and Drug Administration (FDA) recommends that pregnant women consume 200 to 300 mg/day of DHA, which can be achieved by consuming 1 to 2 servings (8 to 12 ounces) of fish per week. However, the average pregnant or lactating woman consumes only 52 mg/day of DHA and 20 mg/day of EPA. For those women with insufficient seafood consumption, alternate sources of DHA include fish oil supplements (150 to 1200 mg/day), enhanced prenatal vitamins (200 to 300 mg/day), DHA-enriched eggs (up to 150 mg/egg), and the plant oils listed previously. Regarding seafood consumption, the American College of Obstetricians and Gynecologists (ACOG) encourages women who are pregnant, planning to become pregnant, or breastfeeding to follow the updated FDA recommendations to avoid fish with the highest mercury concentration, specifically tilefish, shark, swordfish, and king mackerel. They should also limit consumption of white albacore tuna to 6 oz/week.

During the third trimester of pregnancy and the first 2 years of infancy, brain development occurs rapidly, and DHA supplementation has been well studied during this period. The best evidence in support of PUFA supplementation comes from studies that show a positive relationship between PUFA supplementation and neurodevelopmental outcomes in children. Results of randomized, controlled trials have generally supported these findings. For example, one study used cod liver oil supplementation from 18 weeks gestational age until 12 weeks postpartum and found children's mental processing scores at 4 years of age correlated significantly with maternal intake of DHA. However, other randomized, controlled trials have found no difference in cognitive and language scores between offspring of women supplemented with fish oil during pregnancy and those who received placebo. A recently published follow-up study showed that at 18 months of age, children of mothers who were supplemented with 800 mg of DHA during pregnancy showed no difference in mean cognitive, language, or motor scores, although fewer children in the DHA group had delayed development compared with controls.

In addition to the well-studied effects of PUFA on fetal brain development, DHA supplementation during pregnancy has also been promoted as a means to prolong gestation and prevent prematurity. This hypothesis originated after an epidemiologic study compared the diets of women in Denmark to those of the Faroe Islanders, and researchers noted that the seafood-based diet of the island population was associated with higher birthweight (+225 g) at term. This association spawned subsequent research, including one study that found moderate fish intake of up to three meals/week before 22 weeks of gestation was associated with a decreased risk of preterm birth. However, a Maternal Fetal Medicine Network trial did not find evidence that fish oil supplementation decreased the risk of preterm delivery in high-risk patients. Additional studies have shown conflicting results; thus at present, evidence is insufficient to recommend PUFA supplementation as a means to reduce the risk of preterm birth.