Vitamin B-12 is essential for the development and maintenance of a healthy nervous system. Brain development occursprimarily in utero and early infancy, but the role of maternal vitamin B-12 status during pregnancy on offspring cognitivefunction is unclear. In this study we assessed the effect of vitamin B-12 status in well-nourished pregnant women on thecognitive ability of their offspring in a UK birth cohort (ALSPAC). We then examined the association of SNPs in maternalgenes FUT2 (rs492602) and TCN2 (rs1801198, rs9606756) that are related to plasma vitamin B-12, with offspring IQ.Observationally, there was a positive association between maternal vitamin B-12 intake and child’s IQ that was markedlyattenuated after adjustment for potential confounders (mean difference in offspring IQ score per doubling of maternal B-12intake, before adjustment: 2.0 (95% CI 1.3, 2.8); after adjustment: 0.7 (95% CI 20.04, 1.4)). Maternal FUT2 was weaklyassociated with offspring IQ: mean difference in IQ per allele was 0.9 (95% CI 0.1, 1.6). The expected effect of maternalvitamin B-12 on offspring IQ, given the relationships between SNPs and vitamin B-12, and SNPs and IQ was consistent withthe observational result. Our findings suggest that maternal vitamin B-12 may not have an important effect on offspringcognitive ability. However, further examination of this issue is warranted.