Preconceptional intake of which of the following results in decrease in incidence of neural tube defect:
Vitamin A
Folate
Vitamin E
Vitamin C
“Folic acid has been shown to effectively reduce the risk of neural tube defects (NTDs). A daily 4 mg dose is recommended for patients who have had a previous pregnancy affected by neural tube defects. It should be started atleast 1 month (ideally 3 months) prior to pregnancy and continued through the first 6–12 weeks of pregnancy.”
COGDT 10/e, p 197
Remember:

Therapeutic dose of folic acid (to be given in females with previous history of baby with NTD) - 4 mg.
Prophylactic dose of folic acid = 0.4 mg i.e. 400 mg
Duration: It should be started 1 month before conception and continued till 3 months of pregnancy.