Arachnoid cyst at 26 postmenstrual weeks

(A) Axial section showing the sonolucent defect (arrow), which does not communicate with the ventricles. 
(B and C) Transvaginal scans.
(B) This panel is a coronal section showing the cyst (arrow), which pushes the brain parenchyma to one side. However, there is no hydrocephaly.
(C) This panel more precisely shows the location of the cyst, which is in the quadrigeminal cistern, just inferior and posterior to the corpus callosum. No hydrocephaly is evident.