The aim of this study was to determine optimal maternal weight gain in a singleton pregnancy and evaluate the current recommendations. We used a historical prospective design to evaluate the association between pregnancy weight gain and perinatal outcome. All singleton pregnancies without congenital anomalies delivered between 1987 and 1993 at a single institution in New York City were analyzed. After adjusting for the prepregnancy body mass index, we determined the weight gain associated with optimal perinatal outcome. During this 6-year study period, 20,971 pregnant women met the inclusion criteria. Among them, 1,975 (9.4%) had adverse perinatal outcome. Prepregnancy weight and weight gain during pregnancy were strongly associated with adverse outcome. For women of average size, optimal outcome was found in those who gained between 31 and 40 pounds. For women underweight prior to pregnancy, optimal outcome occurred in those who gained 36-40 pounds. For women who were overweight or obese, a gain of 26-30 pounds was associated with optimal outcome. Weight gain during pregnancy is strongly associated with perinatal outcome, independent of important confounding factors, and should be carefully monitored during pregnancy. A randomized controlled trial is required to determine if perinatal and maternal outcome can be improved by advising pregnant women to gain weight using these new ranges rather than the Institute of Medicine's recommendations.