Maternal malaria is associated with reduced birth weight, which is thought to be effected through placental insufficiency, which leads to intrauterine growth retardation (IUGR). The impact of malaria on preterm delivery is unclear. The effects of placental malaria-related changes on birth weight and gestational age were studied in 1177 mothers (and their newborns) from Tanzania. Evidence of malaria infection was found in 75.5% of placental samples. Only massive mononuclear intervillous inflammatory infiltration (MMI) was associated with increased risk of low birth weight (odds ratio ¿OR, 4.0). Maternal parasitized red blood cells and perivillous fibrin deposition both were associated independently with increased risk of premature delivery (OR, 3.2; OR, 2.1, respectively). MMI is an important mechanism in the pathogenesis of IUGR in malaria-infected placentas. This study also shows that placental malaria causes prematurity even in high-transmission areas. The impact of maternal malaria on infant mortality may be greater than was thought previously.