Body mass index--specific weight gains associated with optimal birth weights in twin pregnancies.

To formulate maternal weight gain guidelines, by maternal pregravid body mass index (BMI) status, associated with optimal fetal growth and birth weight in twins. This historical cohort study was based on 2,324 pregnancies with nonanomalous, liveborn twins (4,684 infants) from Ann Arbor, Charleston, Baltimore and Miami. Rates of maternal weight gain and fetal growth were modeled using multiple regression for 0-20 weeks, 20-28 weeks and 28-38 weeks (projected as necessary), controlling for potentially confounding factors. Optimal rates of fetal growth were defined as growth between the singleton and twin 50th percentiles, and optimal birth weights were defined as between the singleton 50th percentile and twin 90th percentile at > or = 36 weeks (2,850-2,950 g). Optimal rates of fetal growth and birth weights were associated with rates of maternal weight gain for underweight women of 1.25-1.75 lb/wk (0.57-0.79 kg/wk) to 20 weeks, 1.50-1.75 lb/wk (0.68-0.79 kg/wk) between 20 and 28 weeks and 1.25 lb/wk (0.57 kg/wk) from 28 weeks to delivery; for normal-weight women, 1-1.5 lb/wk (0.45-0.68 kg/wk) to 20 weeks, 1.25-1.75 lb/wk (0.57-0.79 kg/wk) between 20 and 28 weeks and 1.0 lb/wk (0.45 kg/wk) from 28 weeks to delivery; for overweight women, 1-1.25 lb/wk (0.45-0.57 kg/wk) to 20 weeks, 1-1.5 lb/wk (0.45-0.68 kg/wk) between 20 and 28 weeks and 1 lb/wk (0.45 kg/wk) from 28 weeks to delivery; for obese women, 0.75-1 lb/wk (0.34-0.45 kg/wk) to 20 weeks, 0.75-1.25 lb/wk (0.34-0.57 kg/wk) between 20 and 28 weeks and 0.75 lb/wk (0.34 kg/wk) from 28 weeks to delivery. Optimal rates of fetal growth and birth weights in twins are achieved at rates of maternal weight gain that vary by period of gestation and maternal pregravid BMI status.