Purpose: To evaluate with sonography the normal development of the fetal cerebellum in the second trimester.
Materials and methods: The study included all patients undergoing second trimester genetic amniocentesis who had structurally normal fetuses, in whom the posterior fossa was adequately seen and in whom the karyotype was later determined to be normal. Views of the cerebellum were obtained prospectively, and the inferior vermis was determined to be open or closed at the time of the initial scan. Follow-up was obtained by means of subsequent scans and review of the pediatric record.
Results: Of 897 fetuses that met the inclusion criteria, 147 had an open vermis at initial scanning. At 14 weeks gestation, 56% of fetuses had an open vermis, decreasing to 23% at 15 weeks and 6% at 17 weeks. In utero follow-up sonograms were obtained in 79% of fetuses with an initially open vermis and demonstrated closure in all but one fetus. The remaining 21%, without sonographic follow-up, were normal as newborns.
Conclusion: The prenatal diagnosis of Dandy-Walker variant should not be made before 18 weeks gestation because the development of the cerebellar vermis may be incomplete at that time.