Posterior brain in fetuses with trisomy 18, trisomy 13 and triploidy at 11 to 13 weeks' gestation.

OBJECTIVE To measure changes in the posterior fossa of first-trimester fetuses with trisomy 18, trisomy 13 and triploidy. METHODS Brain stem (BS) diameter and BS to occipital bone (BSOB) diameter were measured in images of the midsagittal view of the face at 11(+0) to 13(+6) weeks from 45 trisomy 18, 21 trisomy 13 and 15 triploid fetuses and compared with values in 162 euploid fetuses. RESULTS In euploid fetuses BS and BSOB diameters increased significantly with crown-rum length and the BS to BSOB ratio decreased. In all three aneuploidies BSOB diameter was significantly higher than in euploid fetuses. In trisomy 18 and trisomy 13, the BS diameter and BS to BSOB ratio were decreased. The BS to BSOB ratio was below the 5(th) percentile in 16 (35.6%), 17 (81.0%) and 5 (33.3%) of trisomy 18, trisomy 13 and triploidy, respectively. In 7 (8.6%) of the aneuploid fetuses there was open spina bifida and in all these cases the BS to BSOB ratio was above the 95(th) percentile. CONCLUSIONS At 11 to 13 weeks' gestation many fetuses with trisomy 18, trisomy 13 and triploidy have measurable abnormalities in the posterior brain.