Ventriculomegaly is a brain condition that mainly occurs in the fetus when the lateral ventricles become dilated. The most common definition uses a width of the atrium of the lateral ventricle of greater than 10 mm.^[1] This occurs in around 1% of pregnancies.^[2] When this measurement is between 10 and 15 mm, the ventriculomegaly may be described as mild to moderate. When the measurement is greater than 15mm, the ventriculomegaly may be classified as more severe.^[3]

Enlargement of the ventricles may occur for a number of reasons, such as loss of brain volume (perhaps due to infection or infarction), or impaired outflow or absorption of cerebrospinal fluid from the ventricles, called hydrocephalus or normal pressure hydrocephalus associated with conspicuous brain sulcus. Often, however, there is no identifiable cause. The interventricular foramen may be congenitally malformed, or may have become obstructed by infection, hemorrhage, or rarely tumor, which may impair the drainage of cerebrospinal fluid, and thus accumulation in the ventricles.^{[citation needed]}

This diagnosis is generally found in routine fetal anomaly scans at 18–22 weeks gestation. It is one of the more common abnormal brain findings on prenatal ultrasound, occurring in around 1–2 per 1,000 pregnancies.^[4] In many cases of mild ventriculomegaly, however, there is resolution of ventriculomegaly during the pregnancy.

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