Malaria in pregnancy has devastating consequences for mother and foetus. WHO recommends intermittent preventive treatment in pregnancy (IPTp) with sulphadoxine-pyrimethamine (SP) for asymptomatic women, but high-level parasite resistance to SP threatens its efficacy. Dihydroartemisinin-piperaquine (DP) has the potential to replace SP for IPTp. However, the DP strategy has not been found to be superior to SP for reducing the incidence of low birthweight (LBW), small-for-gestational age (SGA), or preterm birth. This may be the result of sulphadoxine having antibacterial properties; it is derived from sulphonamide, which have been used for decades to treat curable STIs/RTIs. However, SP is unlikely to be curative of STIs/RTIs, nor highly effective against malaria parasites. Thus, combination treatment that contains a more efficacious antimalarial and a more efficacious anti-STI/RTI may produce better birth outcomes. The investigators will therefore determine whether combining SP with metronidazole (MTZ) or, separately, DP with MTZ can improve birth outcomes more than SP alone, potentially paving the way for integrated control strategies that will reduce the dual burden of malaria and curable STIs/RTIs. This is an individually-randomized, 3-arm, partially-placebo controlled superiority trial comparing the efficacy, safety and tolerance of IPTp-SP versus IPTp-SP with MTZ, or IPTp-DP with MTZ to reduce adverse birth outcomes attributable to malaria and curable STIs/RTIs in 5,436 women in the Nchelenge District of Zambia.