Weight gain

The optimal weight gain during pregnancy is not known precisely. However, the amount of weight gained by the mother during pregnancy has an important bearing on the course and outcome of pregnancy. Adequate weight gain does not necessarily indicate that the diet is nutritionally adequate, but it is associated with a reduced risk of giving birth to a small-for-gestational-age (SGA) or preterm infant.

The desirable weight gain during pregnancy varies among women. The primary factor to consider in making a weight gain recommendation is the appropriateness of the prepregnancy weight for the woman’s height. Maternal and fetal risks in pregnancy are increased when the mother is either significantly underweight or overweight before pregnancy and when weight gain during pregnancy is either too low or too high. Severely underweight women are more likely to have preterm labor and to give birth to LBW infants. Women with inadequate weight gain have an increased risk of giving birth to an infant with intrauterine growth restriction (IUGR). Greater-than-expected weight gain during pregnancy may occur for many reasons, including multiple gestation, edema, preeclampsia, and overeating. Obesity (either preexisting or developed during pregnancy) increases the likelihood of macrosomia and fetopelvic disproportion; operative birth; emergency cesarean birth; postpartum hemorrhage; wound, genital tract, or urinary tract infection; birth trauma; and late fetal death. Obese women are more likely than normal-weight women to have gestational hypertension and gestational diabetes; their risk of giving birth to a child with a major congenital defect is double that of normal-weight women.

A commonly used method of evaluating the appropriateness of weight for height is the body mass index (BMI), which is calculated by the following formula:

BMI=weight/height2

where the weight is in kilograms and height is in meters. Therefore for a woman who weighed 51 kg before pregnancy and is 1.57 m tall:

BMI=51/(1.57)2,or20.7

Prepregnant BMI can be classified into the following categories: less than 18.5, underweight or low; 18.5 to 24.9, normal; 25 to 29.9, overweight or high; and greater than 30, obese (www.nhlbisupport.com/bmi/).

For women with single fetuses, current recommendations are that women with a normal BMI should gain 11.3-15.9 kg during pregnancy, underweight women should gain 12.7-18.1 kg, overweight women should gain 6.8-11.3 kg, and obese women should gain 5.0-9.1 kg (Institute of Medicine, 2009). Adolescents are encouraged to strive for weight gains at the upper end of the recommended range for their BMI because the fetus and the still-growing mother apparently compete for nutrients. The risk of mechanical complications at birth is reduced if the weight gain of short adult women (shorter than 157 cm) is near the lower end of their recommended range.

Pattern of weight gain

Weight gain should take place throughout pregnancy. The risk of giving birth to an SGA infant is greater when the weight gain early in pregnancy has been poor. The likelihood of preterm birth increases when the gains during the last half of pregnancy have been inadequate. These risks exist even when the total gain for the pregnancy is in the recommended range.

The optimal rate of weight gain depends on the stage of pregnancy. During the first and second trimesters, growth takes place primarily in maternal tissue; during the third trimester, growth occurs primarily in fetal tissues. During the first trimester the average total weight gain is only 1 to 2.5 kg. Thereafter the recommended weight gain increases to approximately 0.4 kg per week for a woman of normal weight. The recommended weekly weight gain for overweight women during the second and third trimesters is 0.3 kg and for underweight women is 0.5 kg.

In twin gestations the recommended weight gain for women in the normal BMI category is 16.8 to 24.5 kg, for women who are overweight, 14.1 to 22.7 kg, and for obese women 11.3 to 19.1 kg (Institute of Medicine, 2009). The ideal weight gain for higher multiples is likely to be greater, but no specific recommendations have been issued (Malone & D’Alton, 2009).

The recommended caloric intake corresponds to this pattern of gain. For the first trimester, no increment is necessary; during the second and third trimesters an additional 340 kcal per day and 462 kcal per day, respectively, over the prepregnant intake is recommended. The amount of food that provides the needed increase is not great. The 340 additional kcal needed during the second trimester can be provided by one additional serving from any one of the following groups: milk, yogurt, or cheese (all skim milk products); fruits; vegetables; and bread, cereal, rice, or pasta.

The reasons for an inadequate weight gain (less than 1 kg per month for normal-weight women or less than 0.5 kg/month for obese women during the last two trimesters) or excessive weight gain (more than 3 kg per month) should be evaluated thoroughly. Possible reasons for deviations from the expected rate of weight gain, besides inadequate or excessive dietary intake, include measurement or recording errors, differences in weight of clothing, time of day, and accumulation of fluids. An exceptionally high gain is likely to be caused by an accumulation of fluids, and a gain of more than 3 kg in a month, especially after the twentieth week of gestation, often indicates the development of gestational hypertension.

Hazards of restricting adequate weight gain

Figure-conscious women can have difficulty making the transition from guarding against weight gain before pregnancy to valuing weight gain during pregnancy. In counseling these women the nurse can emphasize the positive effects of good nutrition, as well as the adverse effects of maternal malnutrition (demonstrated by poor weight gain) on infant growth and development. This counseling includes information on the components of weight gain during pregnancy (Table 8-2) and the amount of this weight that will be lost at birth. Because lactation can help to reduce maternal energy stores gradually, this discussion provides an opportunity to promote breastfeeding.

In the United States, 20% of women who give birth are obese (Paul, 2008). However, pregnancy is not a time for weight-reduction. Even overweight or obese pregnant women need to gain at least enough weight to equal the weight of the products of conception (fetus, placenta, and amniotic fluid). If overweight women limit their caloric intake to prevent weight gain, they may also excessively limit their intake of important nutrients. Moreover, dietary restriction results in catabolism of fat stores, which, in turn, augments the production of ketones. The long-term effects of mild ketonemia during pregnancy are not known, but ketonuria has been found to be correlated with the occurrence of preterm labor. The idea that the quality of the weight gain is important should be stressed to obese women (and to all pregnant women), with emphasis placed on the consumption of nutrient-dense foods and the avoidance of empty-calorie foods.

Weight gain is important, but pregnancy is not an excuse for uncontrolled dietary indulgence. Excessive weight gained during pregnancy may be difficult to lose after pregnancy, thus contributing to chronic overweight or obesity, an etiologic factor in a host of chronic diseases, including hypertension, diabetes mellitus, and arteriosclerotic heart disease. The woman who gains 18 kg or more during pregnancy is especially at risk (Box 8-1 and the Critical Thinking/Clinical Decision Making Box).