Smaller: If the posterior cranial fossa pool widening is 10-15mm, it is necessary to closely observe the changes of effusion, measure the size of cerebellar hemispheres, assess the development of cerebellum, perform fetal echocardiography and ultrasound to understand the development of other organs, also need to exclude the presence of viral infection, and perform umbilical cord blood puncture if necessary to exclude fetal chromosomal abnormalities. If there is no abnormality in the fetal brain tissue structure and chromosomal problems, the problem will be less if the fetal ultrasound is reviewed regularly and does not continue to widen. The fetus should be reviewed regularly after birth to prevent continued enlargement to produce hydrocephalus, and if the hydrocephalus is serious, it will affect intelligence; 3. Greater impact: If the posterior cranial fossa pool widening >15mm, it is hydrocephalus and fetal malformation is more likely, and the diagnosis can be clarified by MRI to determine the development of cerebellar hemispheres and the fourth ventricle, except Dandy-Walker syndrome. Severe widening of the posterior cranial fossa pool may lead to fetal hypoplasia of the cerebellar hemispheres and traffic hydrocephalus.