This study examines the effects of maternal prepregnant weight and gestational weight gain on the size, microstructure, and function of the human placenta. Standard gross, histologic, and histomorphometric techniques were used to examine placentas obtained from the deliveries of 77 poor, black 12- to 30-year-old subjects in relation to maternal prepregnant weight and the rate of maternal weight gain during gestation. The weight, volume, and fetal capillary surface area of the placenta increased significantly in relation to both maternal prepregnant weight and the rate of maternal weight gain during gestation. Prepregnant weight was a more important determinant of placental size and fetal capillary surface area than was the rate of maternal weight gain. The rate of maternal weight gain was a more important determinant of the density of fetal capillary tissues within the placenta and of placental resistance to oxygen diffusion than was prepregnant weight. Both maternal prepregnant weight and the rate of maternal weight gain during gestation relate positively to the size of the placenta, but they have different, potentially complementary effects on placental microstructure and function.