Recommended Folic Acid Dose for Primigravida with Prior Anencephaly History
A primigravida with a prior pregnancy affected by anencephaly should take 4,000-5,000 micrograms (4-5 mg) of folic acid daily, making option C (5000 microgram) the correct answer. 1, 2, 3
High-Risk Classification
- Women with a prior pregnancy affected by any neural tube defect, including anencephaly, are classified as high-risk and require substantially higher doses than the standard 400 micrograms recommended for low-risk women 1, 2, 3
- The American College of Medical Genetics explicitly defines high-risk women as those with a prior pregnancy history of neural tube defects, warranting the 4-5 mg daily dose 1
Dosing Algorithm and Timing
- High-dose folic acid (4-5 mg daily) should be started immediately, ideally at least 3 months (12 weeks) before conception, and continued through the first 12 weeks of gestation 1, 2, 3
- After 12 weeks of gestation, the dose should be reduced to 0.4-1.0 mg daily for the remainder of pregnancy to decrease potential health consequences of long-term high-dose folic acid ingestion 1, 2
- Neural tube closure occurs within the first 28 days after conception (approximately 6 weeks from last menstrual period), often before pregnancy recognition, making preconceptional supplementation essential 1, 2
Evidence Base
- The landmark British Medical Research Council study demonstrated that high-dose folic acid supplementation (4.0 mg per day) reduced the risk of recurrent neural tube defects by 70% in women with prior affected pregnancies 4
- Multiple guidelines from the American College of Medical Genetics and the Society of Obstetricians and Gynaecologists of Canada consistently recommend 4-5 mg daily for high-risk women 1, 3, 5
Prescription Considerations
- Prescribe a single prescription-strength 5 mg folic acid tablet rather than multiple over-the-counter multivitamins to avoid excessive intake of other vitamins, particularly vitamin A, which is teratogenic at high doses 6
- Standard over-the-counter prenatal vitamins contain only 0.4-1.0 mg folic acid, which is insufficient for high-risk women 2, 3
Safety Monitoring
- Vitamin B12 deficiency should be ruled out before initiating folic acid doses exceeding 1 mg daily, as high-dose folic acid can mask pernicious anemia while neurological damage progresses 1, 6, 3
- However, vitamin B12 deficiency is uncommon in young women of reproductive age, affecting fewer than 1% of this population 6
Important Clinical Caveats
- Even with adequate folic acid supplementation at 4-5 mg daily, not all neural tube defects can be prevented due to their multifactorial or monogenic etiology, though supplementation prevents approximately 50-72% of cases 1, 2
- Because more than 50% of pregnancies are unplanned, high-risk women should start supplementation immediately rather than waiting for a planned conception 6, 3