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