One of several neural tube defects. The neural tube is a embryonic structure in the developing fetus that forms the spinal cord and brain. Anencephaly is caused by failure of the upper end of the neural tube (the rostral neuropore) to close in the embryo's fourth week of life. This is a fatal malformation and these children die within a few days of delivery. The risk of anencephaly and other NTD's (neural tube defects) can be reduced by taking appropriate amounts of folic acid before conception and throughout pregnancy.