Holoprosencephaly - Rare Birth Defect holoprosencephaly occurs during the first weeks of pregnancy because the fetal brain is not divided as it should be into two cerebral hemispheres and the laterl ventricles ( the cavities of the brain where it circulates the cerebrospinal fluid ) . The lack of division results in a complete or partial union of the cerebral hemispheres and in a partial communication or in a single ventricle , instead of two lateral ventricles