The incidence of ventricular dilatation is about 0.2 per 1,000 births. There is no concept of fetal ventricular widening, but the widening of the ventricles to 7 mm or more requires the physician to be aware of the tendency to develop hydrocephalus and moderate to mild dilatation in the second trimester. The incidence of hydrocephalus and ventricular dilatation in ventricular widening is 6%. Causes: The fetus produces about 650 ml of cerebrospinal fluid per day, and various causes of increased cerebrospinal fluid in the ventricular system and impaired circulation can lead to increased pressure in the ventricular system. The major causes include autosomal invisible and X chromosome recessive gene abnormalities, cerebrospinal fluid circulation pathway disorders, and viral infections. Middle cerebral aqueduct stenosis is the most common cause. Hazards: Mild ventricular widening alone does not cause hydrocephalus in most cases and generally has a good prognosis, but a few cases will have brain damage and abnormal brain development and autosomal invisible gene chromosomal abnormalities. Fetuses with hydrocephalus usually have severe brain tissue damage. Monitoring methods and precautions: (1) The fact that hydrocephalus is not detected in one examination does not mean that hydrocephalus will not occur in the future. (2) It is important to confirm the lateral ventricles in mid-pregnancy. If an echogenic cerebral hemisphere is taken as a ventricle, hydrocephalus will be misdiagnosed. A true lateral ventricle is strongly echogenic, not anechoic. (3) It is recommended to detect high-risk pregnancies with ventricular dilatation and hydrocephalus by color Doppler testing at 22 to 26 weeks and 30 to 34 weeks, respectively, and to increase the number of tests if necessary. (4) Hydrocephalus is also associated with chromosomal and other malformations. (5) Increased lateral ventricular ratio is a strong indicator for the diagnosis of hydrocephalus. lateral ventricular ratio is defined as the ratio of the distance between the lateral ventricular lateral wall of the midline and the distance of the midline from the inner surface of the fetal skull. 16 to 32 weeks fetal biparietal diameter growth of more than 3 mm per week is highly suspected of abnormalities. (6) Even if no fetal brain abnormality is detected during pregnancy, there is no guarantee that the fetus will not develop hydrocephalus after birth. There is no guarantee that hydrocephalus will not occur after birth, because some fetuses may develop hydrocephalus only after a certain period of time.