Anencephaly is what is known as an open neural tube defect. It occurs when the upper part of the neural tube fails to close. This basically means that the skull and overlying scalp are absent and a severely abnormal brain structure is open to outside of the body. There are various sub-groups of anencephaly depending on the involvement of the neck and if it is associated with spina bifida, which is another type of neural tube defect.
Associated abnormalities include hydronephrosis (excessive enlargement of the fluid collecting system for the kidneys), cleft lip and/or palate, diaphragmatic hernia, Exomphalos (see entry Abdominal Exstrophies) and horseshoe kidneys. In addition, spinal abnormalities and abnormal postures of the foot are also observed.