Hydranencephaly - perhaps also rather unkindly termed as a 'water-bag brain' is a condition of the newborn, in which the cerebral hemispheres are essentially absent, due to compromise of the anterior circulation of the brain in utero.   

A range of causes have been inferred ranging from anoxia, infection and maternal exposure to toxins - illicit and otherwise.

The supratentorial brain is replaced with fluid - hence the 'water-bag brain', although the falx cerebri remains, either completely or partially.  Some occipital lobe tissue may be present.

The thalamus and posterior fossa are typically present and normal.

The condition is almost always diagnosed on antenatal ultrasound, providing this service is offered and the mother attends.