Hydranencephaly is a severe congenital condition where most of the cerebral hemispheres are replaced by a membranous sac. Despite the growing amount of case reports, most pathogenic, phenotypic, and prognostic aspects of hydranencephaly remain controversial. By matching the recent literature data with the findings of our own series (four cases: two fetuses at the twelfth gestational week, a 32-year-old man, and a 14-year-old female), we attempted to date back the insult leading to hydranencephaly to understand its pathogenesis and to explain the basis of its protean phenotype. The variable detection of cerebral remnants seems to mirror the developmental pathway of cerebral arteries. Moreover, fetal and postnatal neuroimaging data and histopathologic findings point toward an early bilateral internal carotid artery occlusion, mostly occurring between the eighth and twelfth gestational weeks, as the main pathogenic mechanism of hydranencephaly.