The pathogcnesis of placenta previa was analyzed in a large prospective study that included more than 1000 medical, demographic, hereditary, and postmortem variables. It caused 73 perinatal deaths per 100,000 births. The frequency of the deaths increased with short maternal stature, increasing parity, prior preterm deliveries, and prior perinatal deaths. More male infants died than females. Placentas showed diffuse hyperplastic enlargement of terminal villi, marginal decidual necrosis, and marginal thrombi. These abnormalities were likely related to blood loss and abruption. Both the infants who died and those who survived had a pattern of fetal growth retardation characteristic of undcrnutrjtion. Long-term physical growth and psychomotor development were normal in the surviving infants except for a small excess of neurologic abnormalities.