Placental malaria, in which malarial parasites replicate in the placenta and block the exchange of oxygen and nutrients to the fetus, which is particularly frequent and severe during first pregnancies. Placental malaria can result in low birth-weight, mainly by retarding intrauterine growth, which increases the risk of infant death and disease during the first year of life. Malaria and other tropical diseases are one of the most important causes of spontaneous abortion. Many cases of unexplained pregnancy loss may be due to undiagnosed tropical diseases.