To evaluate the diagnostic value of prenatal ultrasound in fetuses with cerebellomedullary cistern (cisterna magna) dilatation.

Seventy-three fetuses with cisterna magna dilatation (＞10 mm) detected in prenatal ultrasound screening from October 2017 to October 2019 at Peking Union Medical College Hospital, Chinese Academy of Medical Sciences were included. All cases underwent fetal cranial MRI after ultrasound examination, and the examination results were compared to calculate the concordance rate between ultrasound and MRI.

Of the 73 fetuses diagnosed with cisterna magna dilatation by ultrasound, MRI suggested Dandy-Walker malformation in four cases, cerebellar vermis hypoplasia in two, cerebellar hypoplasia in four, posterior fossa arachnoid cyst in eight, Blake's pouch cyst in one, and cisterna magna dilation in 54. Compared with MRI, the diagnostic accuracy of prenatal ultrasound for posterior fossa abnormality was 84.9% (64/73). In this group of patients, 17 patients (23.3%, 17/73) were found to be complicated with other intracranial abnormalities by MRI, including 13 cases (76.5%, 13/17) detected by ultrasound.

Ultrasound has a high diagnostic accuracy for related disorders with fetal cisterna magna dilatation compared with MRI. Major malformations such as Dandy-Walker malformation and cerebellum dysplasia have typical sonographic features and are not easily misdiagnosed, and causion should be taken for disorders such as cerebellar vermis hypoplasia, Blake's pouch cyst, and posterior fossa arachnoid cyst.