A ventricular atrial diameter of more than 10 mm at 20-24 weeks of gestation has been accepted as defining ventriculomegaly because it represents at least 3 standard deviations above the mean (which is roughly 5 mm at 20-24 weeks of gestation). If the diameter is above 15 mm, the ventriculomegaly is considered to be severe. Amongst the most common causes of ventriculomegaly are impaired absorption of CSF, developmental cerebral defects, and destroyed brain tissue.