Williams Obstetrics > Section III. Antepartum >
Chapter 8. Prenatal Care
Nutrition
Meaningful studies of nutrition in human pregnancy are exceedingly difficult to design. For ethical reasons, experimental dietary deficiency must not be produced deliberately. In those instances in which severe nutritional deficiencies have been induced as a consequence of social, economic, or political disaster, coincidental events often have created many variables, the effects of which are not amenable to quantification. Some past experiences suggest, however, that in otherwise healthy women a state of near starvation is required to establish clear differences in pregnancy outcome.
During the severe European winter of 1944–1945, nutritional deprivation of known intensity prevailed in a well-circumscribed area of the Netherlands occupied by the German military (Stein and associates, 1972). At the lowest point during the Hunger Winter, rations reached 450 kcal/day, with generalized rather than selective malnutrition. Smith (1947) analyzed the outcomes of pregnancies that were in progress during this 6-month famine. Median infant birthweights decreased about 250 g and rose again after food became available. This indicated that birthweight can be influenced significantly by starvation during later pregnancy. The perinatal mortality rate, however, was not altered, nor was the incidence of malformations significantly increased. Interestingly, the frequency of pregnancy "toxemia" was found to decline.
Evidence of impaired brain development has been obtained in some animal fetuses whose mothers had been subjected to intense dietary deprivation. Subsequent intellectual development was studied by Stein and associates (1972) in young Dutch adults whose mothers had been starved during pregnancy. The comprehensive study was made possible because all males at age 19 underwent compulsory examination for military service. It was concluded that severe dietary deprivation during pregnancy caused no detectable effects on subsequent mental performance.
Conversely, there is evidence that maternal weight gain during pregnancy influences birthweight. This was studied by Martin and colleagues (2002b) who used birth certificate data for 2001. As shown in Figure 8–7, nearly two thirds of pregnant women gained 26 lb or more. The median weight gain was 30.5 lb. Maternal weight gain had a positive correlation with birthweight, and women with the greatest risk—14 percent—for delivering an infant weighing less than 2500 g were those with weight gains less than 16 lb. This incidence was 20 percent in African-American women who gained 15 lb or less. Cohen and associates (2001) studied more than 4000 pregnant women and concluded that ethnic differences in pregnancy outcomes were not explained by nutritional variations.
Recommendations for Weight Gain
For the first half of the 20th century, it was recommended that weight gain during pregnancy be limited to less than 20 lb (9.1 kg). It was believed that such restriction would prevent pregnancy hypertensive disorders and fetal macrosomia resulting in operative deliveries. By the 1970s, however, women were encouraged to gain at least 25 lb (11.4 kg) to prevent preterm birth and fetal growth restriction, a recommendation that subsequent research continues to support (Ehrenberg and associates, 2003). In 1990, the Institute of Medicine recommended a weight gain of 25 to 35 lb (11.5 to 16 kg) for women with a normal prepregnancy body mass index (BMI). This index is easily calculated using the chart shown in Figure 43–1. Weight gains recommended by the Institute of Medicine (1990) according to prepregnant BMI categories are shown in Table 8–6. The American Academy of Pediatrics and the American College of Obstetricians and Gynecologists (2002) have endorsed these guidelines. Of note, in 2001, almost 1 in 3 women had weight gains outside the Institute of Medicine guidelines (Martin and colleagues, 2002b).
Feig and Naylor (1998) from Canada have challenged recommendations for liberal weight gain in industrial nations. They concluded that these recommendations encourage women to overeat during pregnancy without addressing other causes of low-birthweight infants such as adolescent pregnancy, drug abuse, and heavy smoking. They endorsed the recommendation by the Committee on Medical Aspects of Food Policy (1991) in the United Kingdom that a pregnant woman with a normal BMI should gain 15 to 25 lb during pregnancy.
Disadvantages of excessive maternal weight gain and fetal macrosomia must be considered. Thorsdottir and associates (2002) analyzed pregnancy outcome in relation to weight gain in 615 healthy women with a normal BMI before pregnancy. The frequency of antepartum and intrapartum complications, including fetal macrosomia, was highest among women who gained more than 44 lb (20 kg) during pregnancy. Conversely, these complications were lowest among those whose weight gain was within the range recommended by the Institute of Medicine. Similarly, Rhodes and co-workers (2003) found in their analysis of 1999–2000 United States birth certificate data that excessive weight gain—defined as more than 40 lb—correlated closely with fetal macrosomia.
Weight Retention After Pregnancy
Not all the weight put on during pregnancy is lost during and immediately after parturition (Hytten, 1991). The average-sized normal woman who gains 28 lb (12.5 kg) in pregnancy is about 9 lb (4.4 kg) above her prepregnant weight when discharged postpartum. Schauberger and co-workers (1992) studied prenatal and postpartum weights in 795 women delivered in Wisconsin. Their average weight gain was 28.6 ± 10.6 lb (13.0 ± 4.8 kg). As shown in Figure 8–8, the majority of maternal weight loss was at delivery—about 12 lb (5.5 kg)—and in the ensuing 2 weeks thereafter—about 9 lb (4 kg). An additional 5.5 lb (2.5 kg) was lost between 2 weeks and 6 months postpartum. The average total weight loss resulted in an average retained weight of 3 ± 10.5 lb (1.4 ± 4.8 kg) due to pregnancy. Overall, the more weight gained during pregnancy, the more that was lost postpartum. Parous women retained more of their pregnancy weight, and this is linked to long-term obesity (see Chap. 43, Long-Term Consequences). The effect of breast feeding on maternal weight loss was negligible.
Summary of Weight Gain
Perhaps the most remarkable finding about weight gain in pregnancy is that a wide range is compatible with good clinical outcomes. Moreover, departures from "normal" are nonspecific for any outcome in a given individual.
Recommended Dietary Allowances
Periodically, the Food and Nutrition Board of the Institute of Medicine recommends dietary allowances for women, including those who are pregnant or lactating. Its latest recommendations (2004) are summarized in Table 8–7. Certain prenatal vitamin–mineral supplements may lead to intakes well in excess of the recommended allowances. Moreover, the use of excessive supplements—for example, 10 times the recommended daily allowances—which often are self-prescribed, has led to concern about nutrient toxicities during pregnancy. Nutrients that can potentially exert toxic effects include iron, zinc, selenium, and vitamins A, B6, C, and D. Vitamin and mineral intake more than twice the recommended daily dietary allowance shown in Table 8–7 should be avoided during pregnancy (American Academy of Pediatrics and the American College of Obstetricians and Gynecologists, 2002).