Enlarged fetal cisterna magna: appearance and significance.

The differential diagnosis of a posterior fossa fluid collection in the fetus includes an enlarged cisterna magna, Dandy-Walker syndrome, or a posterior fossa cyst, each of which has differing implications for perinatal management. Causes of an enlarged cisterna magna include cerebellar hypoplasia, communicating hydrocephalus, and normal variation. Cerebellar hypoplasia is a frequent finding in the trisomies and therefore a careful search should be made for associated cardiac anomalies, growth retardation, and polyhydramnios.