This study assesses the effects of prenatal supplementation with United Nations International Multiple Micronutrient Antenatal Preparation (UNIMMAP) compared with iron folic acid (IFA) on average birthweight and incidence of low birthweight. Pregnant women from 78 villages in Niger were included in a cluster-randomized, double-blinded, controlled supplementation trial. Data suggest that prenatal supplementation with multiple micronutrients as part of a prenatal care package in addition to interventions to promote improved maternal prepregnancy nutrition status is an important strategy to increase birthweight and reduce the incidence of low birthweight.