We related the macroscopic and microscopic morphology and the histomorphometry of the placenta to prenatal nutritional supplementation. In the Prenatal Project, a controlled clinical trial, three dietary treatments (supplement, a high-protein beverage; complement, a balanced protein-calorie beverage, or routine vitamin and mineral tablets) were randomly allocated to poor Black pregnant women, and the outcome was assessed. Herein we report the effects on placental morphology and histomorphometry. There were significantly fewer preterm deliveries in the complement group, and this was reflected by an increase in the size of decidual cells, an index associated with placental aging. Several other characteristics of the placentas of the complement group may have been more directly associated with improved perinatal outcome: decreased intervillous fibrin, lower incidence of gross surface infarct, and smaller (and presumably less edematous) cells of the villous stroma, may have mediated increased placental perfusion. There was no evidence of any placental change associated with the increase in very preterm delivery and the highly significant depressed birth weight among preterm deliveries in the supplement group. The significantly lower incidence of meconium staining of Wharton's jelly among controls seems likely to have been a chance finding. While there were several other indices that reflected placental aging, the significantly increased chorioamnionitis, acute funisitis , and acute decidual inflammation among placentas of those who delivered prematurely [the former two associated with very early delivery (less than 35 wk gestation)] were likely to have been involved as causes of premature delivery.