B

Folate-dependent neural tube closure occurs in the first 4 weeks after conception, often before a woman knows she is pregnant; therefore guidelines for women on antiepileptic drugs recommend high-dose folic acid beginning in the pre-conception period—ideally at least 3 months before attempting pregnancy—to lower the drug-related neural tube defect risk [Yerby, 2003, PMID 14504306; Fishman, 2000, PMID 11096742].