When should women take a supplement containing 400 micrograms folate to provide the most protection against neural tube defects?

Women should start taking a supplement containing 400 micrograms of folate at least one month before conception and continue through the first trimester of pregnancy. This timing is crucial, as neural tube development occurs in the early weeks of pregnancy, often before a woman knows she is pregnant. Adequate folate intake during this period significantly reduces the risk of neural tube defects in the developing fetus.