To assess the value of the shape and size of the fourth ventricle in evaluating fetal posterior fossa abnormalities in the second trimester.

Based on ultrasound images, the size of the fourth ventricle was measured, the shape of the fourth ventricle was described, and the data of the fourth ventricle was compared with the previously established normal range. A study was then performed on the clinical data of 58 singleton cases with fetal posterior fossa abnormalities from January 2013 to December 2017 at the Maternal and Child Health Hospital of Sichuan Province. The cases were divided into three groups: group 1, 24 cases with increased fourth ventricle; group 2, 7 with decreased or vanished fourth ventricle; and group 3, 27 with normal fourth ventricle. The pregnancy outcomes of all cases were summarized.

In group 1, there were 15 cases of Dandy-Walker malformation, the transverse diameter of the fourth ventricle was increased, and the transverse diameters were all double standard deviation (2SD) above the value for the same gestational age; there were 9 cases of communicating hydrocephalus, the transverse and anteroposterior diameters, perimeter, and area were all 2SD above the values for the same gestational age. In group 2, 3 had Arnold-Chiari malformationⅡ, 1 had Joubert syndrome, and 3 had rhombencephalosynapsis; the transverse and anteroposterior diameter, perimeter, and area with decreased or vanished fourth ventricle were all 2SD less than the values for the gestational age. In group 3, 10 cases had increased posterior cranial fossa, 3 had posterior cranium arachnoid cyst, 8 had hydrocephalus caused by obstruction above the fourth ventricle, and 6 had Blake's porch cyst; the transverse and anteroposterior diameter, perimeter, and area were all in the normal ranges. In three groups of 58 patients with prenatal diagnosis of posterior fossa abnormalities, after ultrasound or (and) magnetic resonance examination (MRI), 29 pregnant women chose to terminate the pregnancy, and 19 continued to be pregnant until the normal birth of the fetus. Postpartum MRI and autopsy after induction of labor indicated that prenatal ultrasound diagnosis and postnatal examination results were consistent.

The assessment of the size and shape of the fourth ventricle has important value in prenatal ultrasound evaluation of posterior fossa diseases. It is suggested that increased, decreased, or vanished fourth ventricle are not only important diagnostic evidence but also prognostic indicators for posterior fossa diseases. However, our findings still need to further confirmed by multi-center studies with larger samples and wider ranges of gestational weeks.