Absence of the cranial vault and of most or all of the cerebral hemispheres of the brain, a lethal malformation. Anencephaly is due to imper- fect development of the neural tube, the structure that gives rise to the central nervous system, during very early pregnancy. The upper end of the neural tube fails to close. The risk of all neural tube defects, including anencephaly, is decreased if the mother’s diet during pregnancy contains ample folic acid.