Assessment of Fetal Posterior Fossa Anomalies at 11–13+6 Gestational Weeks in the Midsagittal Cranial Plane by Three‐Dimensional Multiplanar Sonography

Objective The aim of this study was to describe the sonographic appearance of posterior fossa anomalies in fetuses at 11–13 +6 weeks' gestation. Methods This prospective study included 60 healthy fetuses and 15 fetuses with an abnormal posterior brain at 11–13 +6 weeks' gestation. All three‐dimensional images were processed using multiplanar image correlation to view the posterior fontanelle in the midsagittal views. The final diagnosis of all fetuses was confirmed using second‐trimester ultrasonography, fetal magnetic resonance imaging, and/or genetic testing. Results The brainstem morphology, fourth ventricle, choroid plexus of the fourth ventricle, vermis, and physiologic Blake pouch were clearly visualized at 11–13 +6 weeks' gestation through the posterior fontanelle from the midsagittal view. Among the 15 fetuses analyzed, two had abnormal brainstem morphology, which was subsequently diagnosed as Walker–Warburg syndrome. The remaining 13 fetuses were diagnosed with posterior fossa cystic malformations (Dandy–Walker malformation, 2 fetuses; Blake's pouch cyst, 2 fetuses; Noonan syndrome, 1 fetus; trisomy 21, 2 fetuses; trisomy 18, 1 fetus; and transient dilatation of the fourth ventricle, 5 fetuses). The extended anterior membranous area and dysplastic vermis were strong markers of Dandy–Walker malformation. In fetuses with Blake pouch cysts, the vermis was visible, with the choroid plexus of the fourth ventricle located backward. Conclusions Sonography enables clear visualization of morphological changes in posterior fossa anomalies at 11–13 +6 gestational weeks. An extended anterior membranous area, dysplastic vermis, and abnormal brainstem morphology are direct signs of early recognition of severe posterior fossa anomalies.