The largest series of normal singleton placental weights was collected in the Collaborative Perinatal Study between the years 1959 and 1966 but values for normal twin placental weights were not published. In our study we examined 787 singleton and 514 twin normal placentas. Placentas with associated conditions known to affect the weights of placentas were excluded. After establishing the normal values for singleton and twin placental weights, we concluded that weight gain of twin placentas appears to accelerate between 24 and 36 weeks but reaches a plateau after 37 weeks, whereas singleton placentas appear to gain weight more uniformly throughout gestation. The mean values of twin placental weights for each gestational age are less than double those of singleton placental weights for the same duration of gestation. Our singleton and twin placentas are heavier than those from previously published data and may reflect a generational or nutritional change over the 30 years since the original numbers were compiled.