Folic acid during pregnancy: dose, timing, and evidence

The short answer: 400–800 mcg/day, starting at least 1 month before conception through first 12 weeks

Folic acid supplementation dramatically reduces the risk of neural tube defects (NTDs) — specifically spina bifida and anencephaly. The evidence is one of the most robust in preventive obstetrics: daily supplementation reduces NTD risk by approximately 70%.

Dose by risk category

• Low-risk pregnancies (standard recommendation): 400 mcg/day from at least 1 month before conception through at least the first 12 weeks of pregnancy.
• Moderate-risk (diabetes, obesity BMI ≥30, anti-epileptic drugs, family history of NTD): 1 mg (1,000 mcg)/day.
• High-risk (previous pregnancy affected by NTD, patient or partner with NTD): 4–5 mg/day (prescription), per your obstetrician.
• Continuation: many guidelines now recommend maintaining 400 mcg/day throughout pregnancy and breastfeeding.

Why timing matters

The neural tube closes between days 21 and 28 of gestation — usually before many women know they are pregnant. That is why folate supplementation must begin before conception, not at the first prenatal visit. Ideally start whenever pregnancy is possible.

Natural folate vs folic acid

• Folic acid (synthetic, in supplements and fortified foods): well absorbed, converted to active 5-MTHF in the body.
• Folate (natural form in leafy greens, legumes, citrus): less stable, lower bioavailability (~50%).
• 5-Methyltetrahydrofolate (5-MTHF) supplements: no conclusive evidence of superiority for general population over folic acid, but often used in women with MTHFR gene variants (ACOG doesn't routinely recommend MTHFR testing).