This is holoprosencephaly, often accompanied by the failure of fetal facial midline structures to form properly. Hence, there are usually midline facial defects (cleft lip, cleft palate, cyclopia, etc) accompanying this condition. The "alobar" form of holoprosencephaly is shown here in which there a single large ventricle, because there is no attempt to form separate cerebral hemispheres. This condition may be associated with trisomy 13. It may rarely occur in association with maternal diabetes mellitus.