aUnaffected women were those without a neural tube defect pregnancy either before or during the particular study, except for Wald et al. and Laurence et al., in which women had at least one neural tube defect pregnancy before the study. bMedian value.
From Wald NJ, Hackshaw AK, Stone R and Sourial NA (1996) Blood folic acid and vitamin B12 in relation to neural tube defects. British Journal of Obstetrics and Gynaecology 103: 319-324.
consistently found lower vitamin B12 levels in affected pregnancies. It is possible that the low levels of vitamin B12 coincide with low levels of folate, although the findings of a case-control study in Dublin suggest that they are independent risk factors and the distribution of the two ingredients in food is dissimilar. In another smaller study, lower levels of vitamin B12 in affected pregnancies were not independent of folate levels. On biochemical grounds, there is so much interaction between the pathways involving both nutrients that it is possible that deficiency of either could affect a common event in the closure of the neural tube. The role of vitamin B12 in NTDs is discussed further later.
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