Purpose or Case Report: Posterior fossa abnormalities are among the most complex and diagnostically challenging findings on foetal MRI. They encompass a wide spectrum of malformations of the cerebellum, brainstem, and fourth ventricle, with variable prognostic implications. Accurate prenatal characterisation is essential for counselling parents, genetic testing and perinatal planning. In this poster we aim to provide a structured, reproducible framework for evaluating foetal posterior fossa abnormalities. A total of 1,400 foetal brain MRI examinations performed over eight years at a tertiary referral centre have been reviewed, identifying all cases with posterior fossa abnormalities. Images were analysed for vermian morphology, cerebellar hemispheric development, fourth ventricular configuration, brainstem integrity, and associated syndromic features. Posterior fossa abnormalities were stratified into five categories: Cerebellar vermis and fourth ventricle malformations, including the recent insights into Dandy–Walker spectrum, Blake’s pouch cyst, isolated vermian hypoplasia, and mega cisterna magna; Cerebellar hemispheric anomalies, such as unilateral/bilateral hypoplasia and cerebellar dysplasia as well as rhombencephalosynapsis; Midbrain–hindbrain junction disorders, including Joubert syndrome and related disorders, and pontocerebellar hypoplasia; Cystic or acquired lesions, including arachnoid cysts and post-ischemic/post-hemorrhagic cystic changes; Syndromic or complex malformations, including Walker–Warburg spectrum and tubulinopathies. The poster highlights refined Dandy–Walker criteria, including inferior-predominant vermian hypoplasia, enlarged tegmentovermian and fastigial recess angles, and inferolateral displacement of the tela choroidea and choroid plexus, distinguishing it from Blake’s pouch cyst and mega cisterna magna. This review illustrates the full spectrum of fetal posterior fossa abnormalities. Annotated MRI cases, schematic diagrams, and a stepwise diagnostic algorithm are provided to enhance interpretative accuracy, reporting consistency, and prenatal counselling.
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