Fetal exposure to valproic acid or carbamazepine increases the risk of neural tube defect (NTD). The risk of a mother having a baby with spina bifida has been estimated at 1-2 p. 100, close to the rate of risk of recurrent cases. No study has evaluated the effect of folic acid in neonates of women treated with valproic acid or carbamazepine although the protective effect against NTD has been proven in other populations. Periconceptional folic acid supplementation, 0.4 to 1 mg/day, for at least one month prior to conception and until the date of the second missed menstrual period or later decreases the incidence of a first occurrence of neural tube defect. Periconceptional folic acid supplementation, 4 mg/day, decreases the recurrence of NTD in women who had previously had a child with NTD. It seems pertinent to recommend periconceptional folic acid supplementation in women treated with carbamazepine or valporic acid. There are very few data in women on which to base a decision to advise taking 4 mg/day (as used in recurrence prevention) or low doses of 0.4 mg/day (used in primary prevention).