## Neural Tube Closure Timeline **Key Point:** The neural tube closes in a bidirectional manner, beginning at the cervical region around day 21–22 (early week 3) and completing by the end of week 4 (approximately day 28). ### Closure Sequence 1. **Primary closure site** — cervical region (around day 21–22) 2. **Rostral closure** — proceeds toward the anterior neuropore (closes ~day 25) 3. **Caudal closure** — proceeds toward the posterior neuropore (closes ~day 28, end of week 4) **High-Yield:** Failure of closure at different timepoints results in distinct defects: | Timing of Defect | Location | Resulting Defect | |---|---|---| | Before day 21 | Entire neural tube | Anencephaly (anterior) or spina bifida (posterior) | | Day 21–25 | Anterior neuropore | Anencephaly | | Day 25–28 | Posterior neuropore | Spina bifida | | After week 4 | Caudal eminence | Sacral agenesis | **Clinical Pearl:** Neural tube defects are among the most common congenital anomalies (1 in 500–1000 births) and are largely preventable with periconceptional folic acid supplementation (400–800 µg daily; 4–5 mg in high-risk pregnancies). **Mnemonic:** **NOPE** — Neural tube closes by end of week **4** (NOPEning after that = defect). 
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