High prevalence of vitamin B12 deficiency in South Asian pregnant women suggests risk for adverse outcomesVitamin B12 deficiency is common among pregnant women in South Asia

Vitamin B12 (cobalamin) deficiency during pregnancy carries a disproportionate burden in South Asia, yet it remains outside India's national antenatal supplementation programme, which continues to prioritize iron and folic acid alone. This narrative review is organized around three questions of programmatic relevance: how large is the burden of maternal vitamin B12 deficiency and what drives it; what maternal and offspring outcomes are associated with low maternal vitamin B12 status, and how strong is the supporting evidence; and what these findings imply for antenatal practice and policy in resource-limited South Asian settings. Following a documented literature search of two databases (January 2000–March 2026), we synthesize evidence on prevalence, dietary and socioeconomic determinants, biological mechanisms, and clinical outcomes, and appraise the strength of inference underpinning each major claim. Pooled global prevalence estimates range from approximately 17% to 25%, whereas Indian and South Asian estimates in pregnancy commonly reach 40–70%. Low maternal vitamin B12 status has been associated predominantly through observational evidence with preeclampsia, pregnancy loss, gestational diabetes, neural tube defects, fetal growth restriction, preterm birth, and impaired offspring neurodevelopment, with hyperhomocysteinaemia, disrupted one-carbon metabolism, and altered methylation as the principal proposed pathways. India's folate-only supplementation policy may contribute to a low-vitamin B12/high-folate imbalance that has been associated with adverse metabolic outcomes in offspring. Randomized evidence, including the recent MATCOBIND trial, consistently demonstrates improvement in biochemical markers of vitamin B12 status; however, evidence for corresponding clinical benefits remains limited. On current evidence, a targeted, risk-based approach to screening and supplementation, rather than universal supplementation, is best supported and can be readily integrated into existing antenatal infrastructure. Priorities for future research include pregnancy-specific diagnostic thresholds, adequately powered trials with long-term follow-up, independent replication of recent intervention findings, and cost-effectiveness analyses in South Asian populations.