Fetal ventriculomegaly is the most common detectable central nervous system abnormality affecting 1% of fetuses and is associated with abnormal neurodevelopment in childhood. Neurodevelopmental outcome is partially predictable by the 2D size of the ventricles in the absence of other abnormalities, while the etiology of the dilatation remains unknown. Fetal cerebral ventriculomegaly is a relatively common finding on second trimester obstetrical ultrasound examination. It is clinically important because it can be caused by a variety of disorders that result in neurological, motor, and/or cognitive impairment. Many cases are associated with other abnormal findings, but in some fetuses, ventriculomegaly is the only abnormality (Isolated ventriculomegaly) Assessment of the width of the atria of the lateral cerebral ventricles is recommended as part of the routine anomaly Scan. To measure the ventricle reliably is particularly problematic when the ventricular measurement is close to the standard threshold of 10 mm, a situation that is often referred to as ‘borderline ventriculomegaly. The consequences of such difficulties in measurement are important since ventriculomegaly is the pivotal finding for diagnosing central nervous system (CNS) pathology, justifying invasive procedures such as fetal karyotyping and generating considerable parental anxiety regarding the postnatal outcome. Even when there is a false positive test result on the initial routine ultrasound examination and the diagnosis is ultimately rejected, the anxiety generated does not dissipate easily. These issues regarding the measurement of the lateral ventricles and the medical and psychological consequences therefore justify revisiting the standardization of the measurement technique. In this lecture, we will focus on the correct ventricular diameter measurement technique and management of ventriculomegaly.