Background Multiple-micronutrient deficiencies often coexist in low- to middle-income countries. They are exacerbated in pregnancy due to the increased demands, leading to potentially adverse effects on the mother. Substantive evidence regarding the effectiveness of multiple-micronutrient supplements (MMS) during pregnancy is not available. Objectives To evaluate the benefits to mother and infant of multiple-micronutrient supplements in pregnancy and assess the risk of excess supplementation and potential adverse interactions between micronutrients. Search strategy We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (30 December 2005). Selection criteria All prospective randomised controlled trials evaluating micronutrient supplementation during pregnancy and its effects on the pregnancy outcome. Data collection and analysis Two review authors independently assessed trial quality and extracted the data. Main results Nine trials (15,378 women) are included. When compared with supplementation of two or less micronutrients or no supplementation or a placebo, multiple-micronutrient supplementation resulted in a statistically significant decrease in the number of low birthweight babies (relative risk (RR) 0.83; 95% confidence interval (CI) 0.76 to 0.91), small-for-gestational-age babies (RR 0.92; 95% CI 0.86 to 0.99) and in maternal anaemia (RR 0.61; CI 0.52 to 0.71). However, these differences lost statistical significance when multiple-micronutrient supplementation was compared with iron folic acid supplementation alone. No statistically significant differences were shown for the outcomes of preterm births and perinatal mortality in any of the comparisons. A number of prespecified clinically important outcomes could not be assessed due to insufficient or non-available data fromthe included trials. These include placental abruption, congenital anomalies including neural tube defects, premature rupture of membranes, preeclampsia, miscarriage, maternal mortality, neurodevelopmental delay, very preterm births, cost of supplementation, side-effects of supplements, maternal wellbeing or satisfaction and nutritional status of children. Authors' conclusions The evidence provided in this review is insufficient to suggest replacement of iron and folate supplementation with a multiplemicronutrient supplement. A reduction in the number of low birthweight and small-for-gestational-age babies and maternal anaemia has been found with a multiple-micronutrient supplement against supplementation with two or less micronutrients or none or a placebo, but analyses revealed no added benefit of multiple-micronutrient supplements compared with iron folic acid supplementation. These results are limited by the small number of studies available. There is also insufficient evidence to identify adverse effects and to say that excess multiple-micronutrient supplementation during pregnancy is harmful to the mother or the fetus. Further research is needed to find out the beneficial maternal or fetal effects and to assess the risk of excess supplementation and potential adverse interactions between the micronutrients.