Objective To determine total homocysteine and folate levels in pregnant women according to vitamin B 12 (B 12 ) levels, and to analyse the impact of maternal B 12 levels on the nutritional status of their newborn babies. Design Cross sectional observational study.Setting Two public hospitals in Jundiai City, São Paulo, Brazil.Sample Sixty-nine pregnant women and their respective newborn babies at the time of delivery.Methods Maternal blood was collected up to 8 hours before delivery. Umbilical cord blood was collected after the expulsion of the placenta. Total homocysteine was measured by high perfomance liquid chromatography, folate by ion capture methodology and B 12 by enzyme immunoassay. Main outcome measures Relationship between low maternal vitamin B12 levels and total homocysteine and folate levels in pregnant women and newborn babies. Results There was a significant correlation between maternal and neonatal B 12 levels, but not between maternal B 12 and neonatal red blood cell (RBC) folate. There was a weak correlation between maternal B 12 levels and neonatal serum folate. However, there were significant correlations between maternal and neonatal total homocysteine levels and between neonatal B 12 and neonatal total homocysteine levels. Mean (FSD) umbilical cord blood B 12 , RBC folate, serum folate and total homocysteine levels were 1.7 F 0.8, 1.8 F 0.8, 2.2 F 0.8 and 0.9 F 0.3 times higher than maternal B 12 , RBC folate, serum folate and total homocysteine values, respectively. Conclusions These data suggest that pregnant women with low B 12 levels are unable to provide the necessary amount of B 12 to their fetuses. The clinical consequences could be the presence of neurological abnormalities as well as the lack of utilisation of homocysteine for methionine transformation.