Malaria in pregnancy is one of the most important preventable causes of low birthweight worlwide and a major cause of severe maternal anaemia contributing to maternal mortality. Intermittent Preventive Treatment (IPT) with sulfadoxine-pyrimethamine (SP) is the currently adopted government recommendation for malaria control during pregnancy in Benin, but the emergence and the spread of resistance to SP justifies the evaluation of alternative anti-malarial drugs. Mefloquine (MQ), which has been proven effective and reasonably safe in this indication, may be an interesting alternative to SP. The aim of this trial is to compare the efficacy and safety of sulfadoxine-pyrimethamine and mefloquine for IPT. It is an equivalent study designed to test the hypothesis that MQ is as efficacious as SP to prevent malaria in pregnancy, and that it could replace SP when resistance of Plasmodium falciparum becomes too elevated. Primary endpoint will be the proportion of infants with low birthweight. Secondary endpoints will be the proportion of mothers with placental plasmodial infection, and the proportion of mothers with anaemia at delivery.