Triplet placentas: reference values for weights.

The occurrence of twins, triplets, and other multiple births increased significantly between 1970 and 2000 in the United States and other industrialized countries. The number of triplet placentas submitted for examination as pathologic specimens has also markedly increased, but no reference values are published for triplet weights. We examined 196 normal triplet placentas. Specimens with associated conditions known to affect the weights of the placentas were excluded. The gestational ages ranged between 20 and 38 weeks. Mean weights for different gestational ages are summarized as follows: 253 g for 20 weeks, 319 g for 22 weeks, 406 g for 24 weeks, 509 g for 26 weeks, 621 g for 28 weeks, 738 g for 30 weeks, 855 g for 32 weeks, 965 g for 34 weeks, 1,065 g for 36 weeks, and 1,147 g for 38 weeks. Weight gain of triplet placentas appears to parallel that of twin placentas. The mean values of placental weights for triplets at each gestational age are less than triple those of singleton weights for the same duration of gestation. The placental weights in multiple gestations do not increase proportionately with the number of fetuses.