Efficacy of chemoprophylaxis in preventing Plasmodium falciparum parasitaemia and placental infection in pregnant women in Malawi.

73 pregnant women in Malawi were given weekly antimalarial chemoprophylaxis under observation and were monitored for Plasmodium falciparum parasitaemia and placental infection. 3 of 19 women (16%) who were parasitaemic at the time they began chemoprophylaxis were infected with chloroquine-resistant P. falciparum. After clearance of initial infections, 25% of the 73 women became parasitaemic while taking prophylaxis and 56% had evidence of active or past placental infection at the time of delivery. None of the women who were parasitaemic at the time of enrollment, and only 11% of those who had breakthrough parasitaemias while taking prophylaxis, had a history of fever and signs or symptoms that they recognized as malaria. Although the density of P. falciparum infection and rates of placental infection appeared to be lower among women taking regular chloroquine prophylaxis, this drug did not prevent P. falciparum infection among pregnant women.