Neural tube defects

—Folic acid supplements in pregnancy have been shown to reduce the incidence of neural tube defects (spina bifida, encephalocoele, and anencephaly) in the fetus and may also reduce the incidence of cleft palate and hare lip. No clear relation exists between the incidence of these defects and folate deficiency in the mother, although the lower the maternal red cell folate (and serum vitamin B12) concentrations even within the normal range, the more likely neural tube defects are to occur in the fetus. An underlying mechanism in a minority of cases is a genetic defect in folate metabolism, a mutation in the enzyme 5, 10 methylenetetra hydrofolate reductase. 

Reff: ABC OF CLINICAL HAEMATOLOGY: Second Edition