Anatomic Approach to Fetal Hydrocephalus.

Hydrocephalus is an imprecise term and refers to the imbalance of brain parenchyma and cerebral spinal fluid in the cranial vault. Ventriculomegaly, or enlargement of the ventricular system, is often the more precise term and is therefore preferred. Appropriate imaging and measurement techniques are critical to detect ventriculomegaly and grade its severity. There is a broad differential diagnosis with resultant highly variable clinical outcomes. Furthermore, some causes have a substantial recurrence risk in future pregnancies. Therefore, accurate diagnosis of the underlying cause is essential for pregnancy management and patient counseling. Understanding ventricular anatomy and correct measurement technique is needed for assessing and grading ventriculomegaly. This is achieved through an anatomically based approach to reach the differential diagnosis, with a focus on key decision points for delineating causes of ventriculomegaly. In particular, the posterior fossa is first assessed for structural or developmental abnormalities; then, midline brain development abnormalities and associated pathologic conditions are reviewed; and last, the cortical mantle is assessed for developmental or destructive processes. In each category, different entities are highlighted, with a particular focus on illustrating the US and MRI findings of the underlying condition and tips to differentiate similar-appearing entities. Ultimately, the informed fetal radiologist can use the proposed algorithmic approach to hydrocephalus to narrow the differential and thus provide vital diagnostic information for pregnancy counseling and management discussions. ^©RSNA, 2025.