The normal width of the posterior cranial fossa is no more than 10 mm, sometimes there is some measurement discrepancy, and a standardized section of the posterior cranial fossa should be measured several times to avoid errors. Each case of widening of the posterior cranial fossa has a different presentation and outcome. There are several outcomes of widening of the posterior cranial fossa: mild simple critical widening, most of the outcomes are good. However, it should be dynamically observed and reviewed, preferably with a chromosomal check. Widening of the posterior cranial fossa has been associated with many chromosomal abnormalities, and further investigations are needed. Some widening of the posterior cranial fossa is due to blake’s cyst, a cystic structure that has no effect on the fetus and can disappear on its own or remain for life. Severe widening of the posterior cranial fossa is often associated with brain malformations, such as cerebellar anomalies, corpus callosum anomalies, cerebral hypoplasia, and many other serious fetal malformations, which have a poor outcome with no hope of cure, and there is no other choice but to induce labor. However, an MRI is done to check the fetal chromosomes before inducing labor. Such malformations should be detected in mid-pregnancy and dealt with promptly and effectively, and should not wait until late pregnancy to be detected.