A Birth defect characterized by failure of the brain and skull to form properly. Parts of the brain and the skull may be missing. This is one of several neural tube defects (NTDs). The neural tube is an embryonic structure in the developing fetus that forms the spinal cord and brain. Anencephaly occurs when the upper end of the neural tube (the rostral neuropore) fails to close in the embryo's fourth week of life. This is a fatal malformation and these children usually die within a few days of delivery. The risk of anencephaly and other NTD's can be reduced by taking appropriate amounts of folic acid before conception and throughout pregnancy. Since dietary sources are not adequate to supply the extra folate recommended during pregnany, a supplement is prescribed.