The public health recommendation to take folic acid in early pregnancy is one of the most evidence-based and cost-effective interventions in preventive medicine. It is also one of the most poorly timed by the majority of women who follow it. The neural tube, the embryonic structure that develops into the brain and spinal cord, closes between days 21 and 28 after conception. Most women do not discover they are pregnant until after this window has closed. Taking folic acid after a positive pregnancy test has already missed the most critical period of neural tube development.
The Evidence Base for Supplementation
The causal relationship between folate deficiency and neural tube defects was established in the landmark MRC Vitamin Study, a randomised controlled trial published in the Lancet in 1991. The trial demonstrated that folic acid supplementation reduced neural tube defect recurrence by 72 percent in women with a previous affected pregnancy. Subsequent trials established primary prevention benefit. Neural tube defects affect approximately 1 in 1000 pregnancies in the UK, and an estimated 70 percent of cases are preventable by adequate periconceptional folic acid intake. Despite this, surveys consistently find that fewer than a third of women begin supplementation before conception.
Dose and Who Needs More
NICE recommends 400 micrograms of folic acid daily for all women planning a pregnancy, beginning at least one month before conception and continuing through the first twelve weeks. This standard dose is appropriate for most women. A higher dose of 5 milligrams daily is recommended for women with a personal or family history of neural tube defects, women with type 1 or type 2 diabetes, women taking valproate or other folate antagonist medications, women with coeliac disease or other malabsorption conditions, and women with a BMI above 30, in whom folate absorption may be less efficient.
Dietary Folate: Complementary, Not Sufficient
Dietary folate from leafy green vegetables, lentils, chickpeas, broccoli, and fortified foods is valuable during pregnancy and throughout life. However, the bioavailability of food folate is considerably lower than supplemental folic acid, and the periconceptional dose recommended to prevent neural tube defects cannot reliably be achieved from diet alone. The UK does not mandate folate fortification of flour, unlike many other countries including the United States and Canada, making supplementation particularly important for UK women. Methyl folate supplements, which bypass the MTHFR enzyme conversion required by standard folic acid, are available and may be preferable for women with confirmed MTHFR variants, though the evidence for their superiority over standard folic acid in clinical outcomes is not yet definitive.
- Begin 400 micrograms of folic acid daily as soon as pregnancy is being considered, ideally at least three months before trying to conceive
- Continue until twelve weeks of gestation: the neural tube is closed by then and the primary period of risk is past
- Ask your GP about 5mg dosing if you have diabetes, take anticonvulsant medication, have had a previous pregnancy affected by a neural tube defect, or have a BMI over 30
- Include folate-rich foods daily throughout pregnancy: dark leafy greens, lentils, asparagus, and chickpeas support ongoing DNA synthesis requirements beyond the neural tube closure window
- If you were not supplementing before conception and have just received a positive test, start immediately: any folate available during neural tube closure is beneficial
Sources
- MRC Vitamin Study Research Group. (1991). Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. Lancet, 338(8760), 131-137.
- NICE. (2008, updated 2021). Antenatal care (NG201). National Institute for Health and Care Excellence.
- Chitayat, D., et al. (2016). Folic acid for pregnant women in Canada: review of dietary intakes and supplementation. Journal of Clinical Pharmacology, 56(2), 170-175.