Open fourth ventricle prior to 20 weeks' gestation: a benign finding?

To evaluate the role of the brainstem-vermis (BV) angle in the diagnosis of fetal posterior fossa abnormalities at 15-18 weeks' gestation.We examined retrospectively three-dimensional (3D) ultrasound volumes acquired at 15-18 gestational weeks in fetuses with normal posterior fossa (controls) and in those with cystic posterior fossa. Whether the fourth ventricle appeared open posteriorly in axial views was noted and the BV angle was measured. A detailed follow-up was obtained in all cases.Of the 139 controls, 46 cases were excluded because of inadequate quality of the 3D volumes. Of the 93 remaining normal fetuses, 84 (90.3\%) had a closed fourth ventricle and a BV angle 45°, significantly increased compared both with that in normal fetuses (P < 0.0001) and with that in fetuses with Blake's pouch cyst (P = 0.004).An open fourth ventricle is found in about 10\% of normal fetuses at 15-18 weeks' gestation. Measurement of the BV angle is useful in such cases, as a value ≥ 45° is associated with a very high risk of severe posterior fossa malformation