anencephaly 1 (Monarch Disease Ontology entry MONDO_0008791) can be described as follows. Anencephaly is a neural tube defect. This malformation is characterized by the total or partial absence of the cranial vault and the covering skin, the brain being missing or reduced to a small mass. Most cases are stillborn, although some infants have been reported to survive for a few hours or even a few days. Also known as: isolated anencephaly/exencephaly.