BACKGROUND: Neonatal mortality is the biggest contributor to global mortality of children younger than 5 years, and low birthweight is a crucial underlying factor. We tested the hypotheses that antenatal multiple micronutrient supplementation would increase infant birthweight and gestational duration. METHODS: We did a double-blind, randomised controlled trial in Dhanusha district, Nepal. Women attending for antenatal care with singleton pregnancies at up to 20 weeks' gestation were invited to participate. Participants were randomly allocated either routine iron and folic acid supplements (control; n=600) or a multiple micronutrient supplement providing a recommended daily allowance of 15 vitamins and minerals (intervention; n=600). Supplementation began at a minimum of 12 weeks' gestation and continued until delivery. Primary outcome measures were birthweight and gestational duration. Analysis was by intention to treat. The study is registered as an International Standard Randomised Controlled Trial, number ISRCTN88625934. FINDINGS: Birthweight was available for 523/600 infants in the control group and 529/600 in the intervention group. Mean birthweight was 2733 g (SD 422) in the control group and 2810 g (453) in the intervention group, representing a mean difference of 77 g (95% CI 24-130; p=0.004) and a relative fall in the proportion of low birthweight by 25%. No difference was recorded in the duration of gestation (0.2 weeks [-0.1 to 0.4]; p=0.12), infant length (0.3 cm [-0.1 to 0.6]; p=0.16), or head circumference (0.2 cm [-0.1 to 0.4]; p=0.18). INTERPRETATION: In a poor community in Nepal, consumption of a daily supplement containing a recommended daily allowance of 15 micronutrients in the second and third trimesters of pregnancy was associated with increased birthweight when compared with a standard iron and folic acid preparation. The effects on perinatal morbidity and mortality need further comparisons between studies. Published online March 3, 2005 http://image.thelancet.com/extras/04art11045web.pdf.