What's your diagnosis in this third trimester fetus?

The fetal lateral and fourth ventricles appear normal in size. However, the cisterna magna (Figure 3, arrow) appears enlarged compared with the normal size and structure of the cerebellum and the vermis. There is also no enlargement of the posterior fossa. Together, these findings suggest a diagnosis of mega cisterna magna. The prognosis in this case is good, and careful follow-up is all that is needed.

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Challenge your diagnostic skills: What's the diagnosis and prognosis based on these images of the fetal brain in a third trimester pregnancy?

Image IQ: Third Trimester Fetal Brain