FETAL 4TH VENTRICLE - US APPEARANCE AND FREQUENCY OF DEPICTION

PURPOSE: To define the normal appearance of the fetal fourth ventricle throughout gestation and ascertain an expected frequency of depiction at different gestational ages. MATERIALS AND METHODS: Three hundred t en consecutive second- and third-trimester fetuses were studied with u ltrasound. The posterior fossa was examined to document the size and a ppearance of the fourth ventricle. RESULTS: The fourth ventricle was s een in 221 of the 310 fetuses (71.3%) and was most consistently demons trated in the middle of the second trimester. At this stage of gestati on, the fourth ventricle was almost always seen when the anatomic feat ures of the posterior fossa were identified. The mean anteroposterior dimension of the fourth ventricle was 3.5 mm +/- 1.3 (standard deviati on), and the mean width was 3.9 mm +/- 1.7. CONCLUSION: The fetal four th ventricle can be seen in most fetuses beginning in the middle of th e second trimester and increases in size with advancing gestation. It can be difficult to depict before the middle of the second trimester a nd late in the third trimester.