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The aetiology and physiology of congenital arachnoid cysts are a source of controversy. We report a case where fetal cerebral ultrasonography shows an extraventricular sonolucent cystic formation after 20 weeks of pregnancy. Ultrasonography provides its topographic relations with adjacent brain structures and is also used to diagnose possible associated malformations. MRI confirms the ultrasonographic findings by investigating cerebral gyri. The rest of the examination involves detection of extracerebral anomalies and a karyotype study. Other differential diagnoses will be considered as a function of the embryological origin and topography of arachnoid cysts. The outcome of these arachnoid cysts depends on the age at the time of diagnosis, their size and their topography. The problem is that hydrocephalus, due to compression of the cerebrospinal fluid drainage pathways, may develop. Treatment, if necessary, is nearly always surgical.