Earlier studies have shown a relationship between maternal vitamin B12 status and birth weight. This study extends those findings directly in terms of neonatal vitamin B12 status and birth weight. One hundred and twelve women were followed from the first trimester of pregnancy and maternal blood was obtained in all three trimesters along with cord blood at birth of their neonates. The maternal and cord serum vitamin B12 concentrations were examined in relation to birth weight. There was a significant correlation between vitamin B12 concentration in maternal antenatal serum during each of the trimesters of pregnancy and cord serum (all P< 0.01). Neonates that were born with lower birth weights (categories of <2500 g and 2500-2999g) had significantly lower mean cord serum vitamin B12 concentrations when compared to those who were > or = 3000g (P = 0.02 and P = 0.05 respectively). A similar, however, non significant trend was observed for antenatal vitamin B12 concentrations at first and third trimesters. Cord serum vitamin B12 concentrations were significantly correlated with birth weight, up to 40 weeks of pregnancy (r=0.28, P=0.01) but not beyond that (> or =40 weeks gestation). Vitamin B12 status in the mother was related to neonatal vitamin B12 status as measured by cord serum vitamin B12 concentration. In addition, low neonatal vitamin B12 concentrations were adversely associated with low birth weights.