Objectives To study the ratio of prenasal thickness (PT) to nasal bone length (NBL) in normal and trisomy-21 fetuses in the second and third trimesters of pregnancy. Methods The PT and NBL were measured retrospectively in 106 normal fetuses (in three-dimensional (3D) volumes) and in 30 fetuses with trisomy 21 (10 on two-dimensional (2D) images and 20 in 3D volumes). Results In normal fetuses the mean PT and NBL increased between 15 and 33 weeks' gestation from 2.3 to 6.1 mm (r = 0.85, P < 0.001) and from 3.3 to 9.6 mm (r = 0.87, P < 0.001), respectively. The PT: NBL ratio was stable throughout gestation, with a mean of 0.61 (95% CI, 0.59-0.63; r = - 0.04, P = 0.7). The 5 th and 95 th percentiles were 0.48 and 0.80, respectively. In trisomy-21 fetuses the mean PT and NBL increased between 14 and 34 weeks from 3.0 to 9.2 mm (r = 0.86, P < 0.001) and from 1.9 to 7.8 mm (r = 0.85, P < 0.001), respectively. The PT: NBL ratio was significantly higher than in normal fetuses (P < 0.001) but also stable throughout gestation, with a mean of 1.50 (95% CI, 1.20-1.80; r = - 0.35, P = 0.07). Twenty-three (77%) of the 30 fetuses with trisomy 21 had a PT above the 95 th percentile and 20 (67%) had an NBL below the 5 th percentile. All the trisomy-21 fetuses had a PT: NBL ratio above the 95 th percentile. When the 95 th percentile of the PT: NBL ratio was used as a cut-off value the detection and false positive rates for trisomy 21 were 100 (95% CI, 89-100)% and 5 (95% CI, 2-11)%, respectively. The positive likelihood ratio was 21.2. © 2011 ISUOG. Published by John Wiley & Sons, Ltd.
de Jong-Pleij, E. A. P., Vos, F. I., Ribbert, L. S. M., Pistorius, L. R., Tromp, E., & Bilardo, C. M. (2012). Prenasal thickness-to-nasal bone length ratio: A strong and simple second- and third-trimester marker for trisomy 21. Ultrasound in Obstetrics and Gynecology, 39(2), 185-190. https://doi.org/10.1002/uog.9047