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A mother is advised folic acid before conception to prevent spinal defects. In week 3–4, the neural plate folds into the neural tube — the future brain and spinal cord. If the tube fails to close at the lower end, the spinal cord stays exposed: spina bifida. Folic acid taken before and during early pregnancy dramatically cuts this risk, because closure happens before most women know they're pregnant. The notochord beneath induces this folding, then mostly disappears — its remnant becomes the nucleus pulposus of intervertebral discs.