The posterior cranial fossa pool is often seen on the prenatal ultrasound checklist and is also called the medullary pool of the cerebellum and the occipital pool, which is located in the lower posterior part of the posterior cranial fossa of the brain. The posterior cranial fossa pool is an important clue to observe the cranial structure of the fetus. The normal width is 2-10 mm, and if the width of the posterior cranial fossa pool is >10 mm, close observation needs to be maintained to be alert to the possibility of fetal malformation. During fetal development, the depth of the posterior cranial fossa pool usually remains within a certain range, usually 2-10 mm, and may increase in width around 32 weeks of gestation and gradually return to normal after 33 weeks. If an abnormal width of the fetal posterior cranial fossa pool is found, one needs to be alert for the possibility of posterior cranial fossa effusion, most often seen in fourth ventricular foramen occlusion syndrome. The fetus is usually associated with a variety of central nervous system developmental abnormalities, such as microcephaly and corpus callosum hypoplasia, and some fetuses may also have extracranial structural abnormalities combined with chromosomal abnormalities. Therefore, if the width of the posterior cranial fossa pool is >10 mm and does not subside after 29-32 weeks of gestation, changes should be closely monitored and the size of the cerebellar hemispheres needs to be measured to assess the development of the cerebellum. At this time, it is necessary to perform a screening test in conjunction with the relevant fetal findings, as well as maternal age and other high-risk factors.