Fetal posterior cranial fossa effusion, seen in Dandy-Walker syndrome, enlarged cerebellar medullary pools and arachnoid cysts in the posterior cranial fossa. It is characterized by cystic dilatation of the fourth ventricle, cerebellar earthworm hypoplasia and obstructive hydrocephalus. Treatment of posterior cranial fossa effusion: If fetal posterior cranial fossa effusion is found, attention should be paid to ultrasound measurement of its depth every 1 to 2 weeks to observe the change of the amount of effusion and its relationship with the gestational week. If the posterior cranial fossa effusion ≤ 10 mm, the size and morphology of the cerebellum are normal. If the posterior cranial fossa effusion gradually disappears, shrinks or has no change, it is a normal variation and has no effect on the perinatal baby; while the posterior cranial fossa effusion > 10 mm does not subside at the peak of the posterior cranial fossa effusion (29-32 weeks), it needs to be closely observed and attention should be paid to measure the size of the cerebellar hemispheres to assess the cerebellar development, and also to observe the fetal heart ultrasound The fetal heart ultrasound should also be observed, as well as the growth and development of other parts of the fetus. When the effusion is >15mm, it should be taken seriously and the possibility of fetal malformation is higher.