First-trimester assessment of nasal bone using retronasal triangle view: a prospective study

To examine the feasibility and accuracy of fetal nasal bone (NB) assessment in the retronasal triangle (RNT) view for aneuploidy screening in the first trimester of pregnancy.

Consecutive women with singleton pregnancies undergoing sonographic screening at 11-13 weeks' gestation were prospectively evaluated. In all cases, assessment of the NB by using the RNT view was attempted and classified as present (if one or both of the NBs were clearly seen) or absent/hypoplastic (if the NB was not visualized or if it was small and less echogenic than the surrounding bones). The detection rate of fetal karyotypic abnormalities by the assessment of the NB in the RNT view was calculated.

In total, 1977 women were scanned. The RNT was successfully examined in 1970 fetuses (99.6%). Fetal outcome was available in 1767 (89.7%) of evaluated cases, and of these, 39 (2.2%) cases of aneuploidy were documented (trisomy 21, n=17; trisomy 18, n=8; trisomy 13, n=5; Turner syndrome, n=5; and triploidy, n=4). The prevalence of absent/hypoplastic NB was 12/1728 (0.7%) in chromosomally normal fetuses and 12/17 (70.6%) in trisomy 21 fetuses. Sensitivity, specificity and positive and negative predictive values of absent/hypoplastic NB for trisomy 21 were 70.6%, 99.3%, 50.0% and 99.7%, respectively. The positive and negative likelihood ratios of NB assessment were 101 (95% CI, 53-193) and 0.3 (95% CI, 0.14-0.62), respectively.

The RNT view is a useful technique for assessing the NB during the first trimester of pregnancy. With this new approach, performance of absent/hypoplastic NB as a marker of aneuploidy, mainly trisomy 21, appears to be similar to that previously reported by using the mid-sagittal plane.