Anencephaly is a rare congenital neural tube defect characterized by the absence of a major portion of the brain, skull, and scalp. It occurs when the upper part of the neural tube, which forms during early pregnancy, fails to close completely. Anencephaly is a fatal condition, and babies born with this condition usually do not survive more than a few hours or days after birth. The exact cause of anencephaly is unknown, but it is believed to be caused by a combination of genetic and environmental factors.