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Background Vitamin B 12 deficiency is common in populations with limited animal-source foods and has been linked to delayed infant neurodevelopment and adverse pregnancy outcomes. Evidence on the benefits of maternal B12 supplementation for improving infant neurodevelopment remains mixed, particularly in low-income and middle-income countries where deficiency is prevalent.Methods This double-blind, randomised controlled trial was conducted in two tertiary maternity care centres in India and Nepal. Pregnant women in their first trimester, following a vegetarian diet, were enrolled and randomised to receive either a daily oral supplement of 250 µg (group A) or 50 µg (group B) of methyl-cobalamin from enrolment to 6 months post partum. The primary outcomes were infant neurodevelopment assessed at 9–12 months using the Development Assessment Scale of Indian Infants and biochemical B 12 status in mothers and infants measured through blood tests.Results 531 mothers completed the study (group A n=255; group B n=276). There were no significant differences in baseline characteristics between mothers at both centres or in groups A and B. Mental developmental quotients (DQs) were higher in the infants of group A: 103.7 (7.7) than group B: 101.7 (8.8); p=0.008). This corresponds to a mean difference of 7.8 centiles (p=0.007). Mean motor DQs were not significantly different between the groups. Maternal vitamin B 12 levels rose from the first to third trimester in both groups, with a larger increase in group A (104.9 pg/mL (SD 159.1)) than group B (47.5 pg/mL (SD 118.0)), p<0.001. Holotranscobalamin levels improved similarly (p<0.001). All infant levels of vitamin B12 were within the normal range. Newborn anthropometry, APGAR scores and morbidity profiles were similar in both groups (p>0.05). Serum ferritin, vitamin D and folate were similar (p>0.05).Conclusions Daily supplementation with 250 µg of vitamin B 12 during pregnancy in vegetarian mothers significantly improved infant mental DQ and maternal B12 status compared with a 50 µg dose.