WEIGHT GAIN AND PREGNANCY
An infant's birth weight is affected by many factors, including the mother's prepregnancy weight-for-height value and weight gain during pregnancy. Prepregnancy weight-for-height value is expressed as body mass index (BMI). BMI is defined as weight in kilograms divided by the square of height in meters. In 1959, the Metropolitan Life Insurance Company defined its weight-for-height standards by BMI. These standards are in common use today.
Underweight is defined as a BMI of less than 19.8. Normal weight is defined as a BMI of 19.8-26, which corresponds to between 25% and 75% of the expected weight-for-height value. Overweight is defined as a BMI of 26-29. Lastly, obesity is defined as a BMI that exceeds 29. Birth weight is affected by prepregnancy BMI independent of actual weight gain during pregnancy. Women who are underweight are at increased risk for low birth weight babies; women who are overweight or obese are at increased risk for macrosomic infants. Macrosomia is variably defined as weight exceeding 4000 g, 4500 g, or the 90th percentile. Macrosomic infants are at increased risk for shoulder dystocia and brachial plexus injuries.
Morbid obesity is defined by a BMI exceeding 35. Morbidly obese patients are at increased risk for preeclampsia, nonreassuring fetal heart tracings, meconium aspiration, late intrauterine fetal death, and early neonatal death (Cedergren, 2004).
Of course, birth weight is also affected by weight gain during pregnancy. Although weight should be gained throughout pregnancy, it is most critical in the second trimester. Even if overall weight gain is poor, birth weight is usually acceptable with appropriate second-trimester weight gain. The following table relates low birth weight to both prepregnancy weight and pregnancy weight gain.
Rates of Low Birth Weight (<2500 g) by Weight and Weight Gain
Pregnancy Weight Gain, lb______________Prepregnancy Weight, lb
______________________________<110__110-129__130-149__>150
<16____________________________30%___ 20%____15%_____7-8%
25-35__________________________7-8%___6-7%___4-5%_____3-4%
>35____________________________5-6%___3-4%___3-4%_____3-4%
In 1990, the Institute of Medicine issued recommendations for weight gain during pregnancy (Institute of Medicine, 1990). These recommendations are based on prepregnancy BMI. Women who are underweight are advised to gain a total of 12.5-18 kg (28-40 lb). This translates to 0.5 kg/wk in the second and third trimesters. Women of normal weight are advised to gain a total of 11.5-16 kg (25-35 lb), or 0.4 kg/wk, in the second and third trimesters. Women who are overweight or obese should limit their weight gain to 7-11.5 kg (15-25 lb), or 0.3 kg/wk, in the second and third trimesters.
Dieting during pregnancy is never recommended, even for patients who are morbidly obese. Severe restriction of energy (caloric) intake is associated with a 250-g decrease in average birth weight. Because of the expansion of maternal blood volume and construction of fetal and placental tissues, some weight gain is essential for a healthy pregnancy.
Weight gain within these parameters is associated with a lower rate of cesarean delivery, fewer infants with growth restriction or macrosomia, and a decreased incidence of postpartum obesity. Nevertheless, only 30-40% of pregnant women achieve appropriate weight gain (Hickey, 2000). Further evaluation is needed if weight gain is persistently slow or does not equal 10 lb by mid pregnancy.
DIET IN PREGNANCY
The demands of pregnancy necessitate additional dietary requirements. Obviously, additional energy (caloric) intake is required to support recommended weight gain. Because energy requirements in pregnancy are increased by 17% over the nonpregnant state, a woman of normal weight should consume an additional 126 kJ/d (300 kcal/d); however, this energy should be of high nutrient density. Nutrient density reflects the amount of protein, vitamins, and minerals per 418 kJ (100 kcal) of food.
Protein should comprise 20% of a normal pregnancy diet. The recommended daily allowance (RDA) in pregnancy is 60 g. Fortunately, most American diets already contain more than enough protein. Pregnant women should be aware that many animal sources of protein are very high in fat and might contribute to excessive weight gain; therefore, animal proteins should be taken sparingly. Fat should only comprise 30% of a normal pregnancy diet. Carbohydrates should comprise the remaining 50%.
A sample diet for normal pregnancy is based on the food pyramid and should include 6-11 servings of grains; 3-5 servings of vegetables; 2-4 servings of fruit; 3-4 servings of dairy; 2-3 servings of meats, beans, or nuts; and 1 serving of sweets. Total energy intake should vary by BMI, but the average recommendation is 10,460 kJ/d (2500 kcal/d).
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