Dilated fourth ventricle in fetuses with trisomy 18, trisomy 13 and triploidy at 11-13 weeks' gestation.

OBJECTIVE: To determine if in fetuses with aneuploidies the diameter of the fourth cerebral ventricle at 11-13 weeks' gestation is different from euploid fetuses. METHODS: The fourth ventricle at 11-13 weeks' gestation was assessed in 62 cases of trisomy 21, 32 of trisomy 18, 10 of trisomy 13, and 12 of triploidy and compared to 410 normal euploid fetuses. Transvaginal sonography was carried out and 3D brain volumes were acquired. The fetal head was assessed in an axial plane and the diameter of the fourth ventricle was measured. Values in aneuploid and euploid fetuses were compared. RESULTS: The diameter of the fourth ventricle in trisomy 18, trisomy 13 and triploidy, but not in trisomy 21, was significantly higher than in euploid fetuses. In the euploid fetuses the median diameter of the fourth ventricle was 1.9 mm and the 95th percentile was 2.5 mm. The measurements were above the median and the 95th percentile in 25 (78.1%) and 17 (53.1%) cases of trisomy 18, in 10 (100%) and 8 (80.0%) of trisomy 13, and in 10 (83.3%) and 10 (83.3%) of triploidy. CONCLUSIONS: In trisomy 18, trisomy 13 and triploidy the diameter of the fourth ventricle at 11-13 weeks' gestation is increased.