Malaria during pregnancy may result in fetal exposure to malaria when parasites are transmitted across the placenta. To document the rate of transplacental passage of Plasmodium falciparum and to identify the risk factors for congenitally acquired malaria infection, we examined umbilical cord blood for malaria parasites from 2,080 newborn infants born to mothers enrolled in a study of malaria prophylaxis during pregnancy. Cord blood parasitemia was detected in 140 (6.7%) newborn infants with a geometric mean density of 187 parasites/microliter (range 12-99, 752 parasites/microliter). The likelihood of umbilical cord blood parasitemia was closely linked to the parasite density of placental malaria infection and the density of maternal peripheral blood parasitemia at the time of delivery; all babies born to women with both placental and peripheral blood parasitemia densities > or = 10,000/microliter had cord blood parasitemia. In a multivariate logistic regression model, male sex, premature delivery, and placental and maternal peripheral blood malaria parasitemia were independently associated with a baby being born with umbilical cord blood parasitemia. In this setting, highly endemic for malaria, transplacental transmission of malaria from infected placentae occurs frequently and is directly related to the density of maternal malaria infection.