Subnormal cerebellum in fetuses with spina bifida.

The results of a prospective ultrasound study of the posterior fossa structures in 19 fetuses with spina bifida are reported. The transverse cerebellar diameter is below the normal limits and the cisterna magna is obliterated in all cases. Decreased cerebellar size and failure to visualize the cerebellum appear to be specific findings for spina bifida since they are not present in a group of 17 fetuses with isolated hydrocephalus. The results of this study indicate that ultrasound investigation of the posterior fossa may prove useful in the prenatal diagnosis of spina bifida.