With the current advances in prenatal ultrasound technology, the sonographer may be able to provide early detection of previously occult disease during the prenatal period. However, with this comes the confusion of follow-up evaluation. Hydrocephalus is usually caused by obstruction of the flow of cerebrospinal fluid and includes both intraventricular (non-traffic) and extraventricular (traffic) types. Ventricular dilatation is a manifestation of hydrocephalus, defined as axial measurement of the posterior horn of the lateral ventricle greater than or equal to 10 mm without regard to gestational age. Etiology: stenosis of the midbrain conduit, spinal cord bulge, intrauterine infections (cytomegalovirus, toxoplasmosis, syphilis), agenesis of the corpus callosum, X-linked hydrocephalus syndrome, intracranial hemorrhage, Dandy-Walker malformation and intracranial tumors. Of the ventricular dilatation, 40% were combined with malformations of the central nervous system or beyond, and 12% were associated with chromosomal abnormalities. Cerebrospinal fluid (choroid plexus 50%; cerebral capillaries 50%) Lateral ventricle – interventricular foramen – third ventricle – fourth ventricle – aqueduct – median foramen of the fourth ventricle or lateral foramen of the fourth ventricle Prognosis: 2.5-4.5% will progress; 54%-84% will be combined with other anomalies; the main influence on prognosis is whether other anomalies are combined. In a group of cases with a critical range of ventricular dilatation, 93% had normal neurodevelopment, of which the chance of epilepsy was about 10%. The risk of recurrence in the next child was about 4%.