Enlargement of the fetal posterior fossa - What is the significance?

The posterior fossa may be enlarged due to: mega cisterna magna, arachnoid cyst or Dandy Walker malformation. These anomalies can be identified during routine ultrasound screening performed in the beginning of the second half of pregnancy (22nd week). The outcome of these findings is not clear. Therefore, it is difficult to advise the parents how to manage the pregnancy after the discovery of this anomaly. Most of the studies report a good prognosis in cases of mega cisterna magna or arachnoid cyst, as long as this finding is isolated. A Dandy-Walker malformation or non-isolated enlargement of the posterior fossa entail a guarded prognosis. Further studies are needed in order to enable better counseling.