Fetal fourth 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 ten consecutive second- and third-trimester fetuses were studied with ultrasound. The posterior fossa was examined to document the size and appearance of the fourth ventricle.

RESULTS

The fourth ventricle was seen in 221 of the 310 fetuses (71.3%) and was most consistently demonstrated in the middle of the second trimester. At this stage of gestation, the fourth ventricle was almost always seen when the anatomic features of the posterior fossa were identified. The mean anteroposterior dimension of the fourth ventricle was 3.5 mm +/- 1.3 (standard deviation), and the mean width was 3.9 mm +/- 1.7.

CONCLUSION

The fetal fourth ventricle can be seen in most fetuses beginning in the middle of the second trimester and increases in size with advancing gestation. It can be difficult to depict before the middle of the second trimester and late in the third trimester.