Neural Tube Closure Timing
Neural tube closure occurs by approximately 4 weeks' gestation (28 days after conception), which is often before a woman even recognizes she is pregnant. 1
Precise Timeline
The embryonic neural tube must close properly by approximately 4 weeks' gestation to prevent neural tube defects (NTDs) such as anencephaly, spina bifida, and encephalocele. 1
More specifically, neurulation occurs between the 17th and 30th day after ovulation, representing a well-defined critical period during early embryogenesis. 2
The anterior neural tube closes first (preventing anencephaly), followed by closure along the rest of the neural tube. 1
Critical Clinical Implications
Why This Timing Matters for Prevention
Because neural tube closure occurs within the first 28 days after conception—often before pregnancy recognition—folic acid supplementation must begin BEFORE conception to be maximally effective. 1
For women at standard risk, 400 μg (0.4 mg) of folic acid daily should be taken throughout reproductive years, starting at least 1-2 months before conception and continuing through the first trimester. 1
For women at high risk (prior NTD-affected pregnancy, family history, or taking antiepileptic medications), 4-5 mg of folic acid daily is recommended, beginning at least 3 months before conception. 3, 4
Common Pitfall to Avoid
Starting folic acid supplementation only after pregnancy confirmation is too late for optimal NTD prevention, since neural tube closure is already complete or nearly complete by the time most women miss their first period and confirm pregnancy. 1
Many pregnancies are unplanned (over 50%), making preconceptional supplementation throughout reproductive years the most effective public health strategy. 1
Pathophysiology
Neural tube defects result when the embryonic neural tube fails to close properly during this 4-week window, with clinical consequences depending on the site and severity of the defect. 1, 5
Approximately 80% of NTDs are "open" defects (exposing neural tissue), while 20% are "closed" or skin-covered defects. 1, 6
Failed closure of the anterior neural tube causes anencephaly, which is lethal and usually results in miscarriage, stillbirth, or early neonatal death. 1