II. Neural Tube Defects
Neural tube defects, which account for most CNS malformations, are due to failure of a portion of the neural tube to close, or reopening of a region after successful closure. Recall that the neural tube forms gradually from the neural plate by "zippering" in caudal and cephalad directions from the cervical region.
The last portions of the neural tube to close are the anterior and posterior neuropores. The latter closes at about the 26th day of gestation. Midline defects, which are directly related to the lack of closure of the neural tube, are among the most common of central nervous system malformations. These defects, called dysraphic states, are probably related to both genetic and environmental causes. Since amniotic fluid contains increased alpha-fetoprotein content with "open" neural tube closure defects, prenatal diagnosis is possible.
Folic acid supplementation before pregnancy and during its early stages markedly reduced the risk of neural tube defects in newborns. This finding led to the recommendation that women planning to become pregnant should take folic acid supplements.