The incidence of fetal posterior cranial fossa effusion was 2.0%, the earliest gestational week in which the first detection of posterior cranial fossa effusion was made was 22 weeks, and the latest was 41 weeks, with an average of 31±4 weeks, and posterior cranial fossa effusion was most common in the 29-32 weeks of pregnancy, with the highest amount of effusion in the 29-32 weeks of pregnancy. The amount of fluid in the posterior cranial fossa was 10-14 mm, and the incidence of poor perinatal outcome was 4.0%, 7.6% and 83.3% in the cases of <10 mm, 10-14 mm and ≥15 mm respectively. Therefore, those with more than 10 mm of fluid need to be closely followed up regularly, and the ultrasound should be repeated once every two to four weeks. Therefore, those over 10 mm need to be followed up regularly and repeat the ultrasound every 2-4 weeks. If the size is greater than 15mm, the possibility of fetal anomalies is high and should be taken seriously.