Prepregnancy body mass index and pregnancy weight gain: associations with preterm delivery. The NMIHS Collaborative Study Group.

OBJECTIVE: To examine associations between rate of pregnancy weight gain and preterm delivery among women of varying prepregnancy body mass indices (BMI).

METHODS: Subjects were 3511 mother-infant pairs from the 1988 National Maternal and Infant Health Survey. Prenatal weight measured between 14 and 28 weeks' gestation was used to calculate rate of pregnancy weight gain for each woman. Weight gain (lb/week) was categorized as low (under 0.5), average (0.5-1.5), or high (above 1.5). Prepregnancy BMI was calculated as weight divided by height in (kg/m(2)) and categorized as low (under 19.8), average (19.8-26.0), and high (above 26). Delivery before 37 weeks' gestation was considered preterm. Associations between BMI, weight gain, and preterm delivery were examined before and after exclusion of medically indicated preterm deliveries and pregnancies complicated by maternal medical conditions potentially related to weight gain or fetal growth restriction. Associations were expressed as odds ratios (OR) adjusted for several potential confounding factors.

RESULTS: Women with low pregnancy weight gain were at increased risk of preterm delivery. The magnitude of risk varied according to a woman's prepregnancy BMI. After all exclusions and adjustments for confounders, ORs, and 95% confidence intervals (CI) for low pregnancy weight gain were 6.7 (1.1, 40.6) for underweight women, 3.6 (1.6, 8.0) for average-weight women, and 1.6 (0.7, 3.5) for overweight women compared with average-weight women with average pregnancy weight gain.

CONCLUSIONS: Low weight gain in pregnancy was associated with increased risk of preterm delivery, particularly if women were underweight or of average weight before pregnancy.