The ventricles are fluid-filled spaces in the brain. The two major ventricle are the lateral ventricles. A lateral ventricle that's over 10 mm in size is usually considered enlarged (ventriculomegaly). Fetuses with ventriculomegaly usually require additional ultrasound examinations to see if there are other anomalies. Fetuses with additional abnormal findings are at greater risk for having a chromosomal abnormality. Mild, isolated ventriculomegaly that is unilateral (on one side of the brain only) appears to have little effect on the future development of the baby.