Prenatal ultrasound diagnoses or suggests that fetal critical value ventricular dilatation is very common, and its main cause is caused by impaired cerebrospinal fluid reflux. The ventricles may also be passively dilated if the brain is abnormal and the cerebral cortex is underdeveloped. However, some ventricular dilatation is of unknown cause and some can be normal variants. The incidence of ventricular dilatation in the fetus is 0.39%. Grading of ventricular dilatation: 1. severe ventricular dilatation (hydrocephalus): lateral ventricular dilatation more than 15 mm; 2. mild ventricular dilatation 10-12 mm; 3. critical value ventricular dilatation: lateral ventricular dilatation 8-10 mm. Most scholars consider fetuses within the range of lateral ventricular width not exceeding 9.9 mm at any time to be normal variants. If fetal lateral ventricular dilatation ≥16 mm or bilateral ventricular dilatation is detected by ultrasound screening, MRI is recommended to determine the diagnosis of fetal central nervous system disease; simple lateral ventricular dilatation with ventricular dilatation ≤12 mm does not require MRI, but can be observed dynamically by ultrasound, and then MRI is performed when an increasing trend of fetal ventricular dilatation is detected to clarify the diagnosis.