Background: Too little or too much weight gain during pregnancy is associated with adverse fetal outcomes. In general, mothers are advised to gain at least 15 lb (6.8 kg). Current guidelines provide an optimal range of weight gain based on maternal prepregnancy body mass index (BMI), but do not specify an upper limit of weight gain for obese mothers (i.e., those with a BMI of 30 kg per m² or greater). Kiel and colleagues used an obstetric database from a large state to study the relationship of weight gain and pregnancy outcomes in obese women.

The Study: The study included all singleton deliveries after 37 weeks' gestation between 1990 and 2001 in which the mother's prepregnancy BMI was 30 kg per m² or greater. For each participant, pregnancy weight gain was calculated from obstetric records. Pregnancy outcomes assessed included preeclampsia, cesarean delivery, small for gestational age (SGA) infant, and large for gestational age (LGA) infant. Demographic data collected included maternal age, ethnicity, education level, tobacco use, and enrollment in a public assistance program.

Results: During pregnancy, 23 percent of the 120,170 mothers gained less than 15 lb, 31 percent gained 15 to 25 lb (6.8 to 11.3 kg), and 46 percent gained more than 25 lb. For all levels of obesity, the risk of pre-eclampsia, cesarean delivery, SGA, or LGA was related to weight gain during pregnancy. The estimated minimal risk for any outcome was for a weight gain of 10 to 25 lb (4.5 to 11.3 kg) for a BMI of 30 to 34.9 kg per m²; a gain of 0 to 9 lb (0 to 4.1 kg) for a BMI of 35.0 to 39.9 kg per m²; and a weight loss of 0 to 9 lb for a BMI of 40 kg per m² or greater. After adjustment, mothers who gained less than 15 lb had significantly lower odds of preeclampsia, cesarean delivery, and LGA birth, but higher odds of SGA birth, than mothers who gained 15 to 25 lb. Women who gained more than 25 lb had higher odds of preeclampsia, cesarean delivery and LGA birth, but lower odds of SGA birth.

The authors calculated numbers needed to treat (NNT) (i.e., the number of women who gained less than 15 lb) to prevent one adverse event for each class of obesity. The NNTs ranged from 13 to prevent one case of LGA in class II (BMI of 35 to 39.9 kg per m²) or class III (BMI of 40 kg per m² or higher) obese mothers, to 52 to prevent one case of SGA in the heaviest mothers.

Conclusion: The authors recommend that obese women have more favorable outcomes with limited (up to 15 lb) or no weight gain during pregnancy.

editor's note: An editorial and two other articles in the same issue of the journal address the growing problem of pregnancy in obese mothers and optimal pregnancy weight gain. DeVader and colleagues demonstrated that in non-obese mothers, weight gain of less than 25 lb was associated with lower odds for many adverse outcomes, but increased odds of SGA infants.¹ Cedergren's study calculates the optimal gestational weight gain for each class of prepregnancy BMI as 9 to 22 lb (4.1 to 10.0 kg) for a BMI up to 20 kg per m²; 5 to 22 lb (2.3 to 10.0 kg) for a BMI of 20 to 24.9 kg per m²; less than 20 lb (9.1 kg) for a BMI of 25 to 29.9 kg per m²; and less than 13 lb (5.9 kg) for a BMI of 30 kg per m² or greater.²

The editorial stresses two main problems—more obese mothers beginning pregnancy and almost one half of all mothers gaining more than the recommended weight levels during pregnancy.³ In addition to the obvious relationship with adverse pregnancy outcomes, the editorial describes the increasing evidence that many adult health problems may originate in prenatal life (fetal programming). The weight of women of reproductive age and those already pregnant must be a priority.—a.d.w.