Appearance of an interhemispheric cyst associated with agenesis of the corpus callosum.

We describe a fetus with agenesis of the corpus callosum (ACC) and Dandy-Walker malformation that developed a frontal paramidline cyst late in gestation. The interval appearance of the cyst occurred in concert with increasing size of the lateral ventricles, which supports the hypothesis that cysts associated with ACC can develop with increasing intraventricular pressure. Recognition of the potential for a changing appearance of neurologic abnormalities is important to providing appropriate patient counseling.