Dilatation of the fetal cerebral ventricles (ventriculomegaly) is a generic sonographic sign common to several pathological entities carrying different prognoses. The main causes of fetal ventriculomegaly are aqueductal stenosis, Chiari II malformation, Dandy-Walker complex and agenesis of the corpus callosum. Ventriculomegaly is easily recognized by ultrasound by measuring the atrial width. This simple measure allows the recognition of mild forms of ventricular dilatation and is used as a screening method for ventriculomegaly. However, although the diagnosis of ventriculomegaly is easy, the prenatal identification of the cause of ventricular dilatation is a more difficult task. To this end, the evaluation of the posterior fossa in association with the visualization of the corpus callosum is a useful landmark. Research into the cause of ventriculomegaly is clinically useful, since the prognosis mainly depends on the etiology and on the presence of associated anomalies. In this article the role of prenatal sonography in recognizing the cause of ventriculomegaly and the prognostic value of the prenatal sonographic findings are discussed.