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Prenasal thickness‐to‐nasal bone length ratio: a strong and simple second‐ and third‐trimester marker for trisomy 21
Author(s) -
De JongPleij E. A. P.,
Vos F. I.,
Ribbert L. S. M.,
Pistorius L. R.,
Tromp E.,
Bilardo C. M.
Publication year - 2012
Publication title -
ultrasound in obstetrics and gynecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.202
H-Index - 141
eISSN - 1469-0705
pISSN - 0960-7692
DOI - 10.1002/uog.9047
Subject(s) - trisomy , medicine , fetus , percentile , gestation , nasal bone , aneuploidy , pregnancy , nuclear medicine , down syndrome , obstetrics , gynecology , anatomy , biology , biochemistry , statistics , genetics , mathematics , gene , chromosome , psychiatry
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. Conclusions The PT : NBL ratio is stable in the second and third trimesters of pregnancy in both normal and trisomy‐21 fetuses, but all trisomy‐21 fetuses in this series had a PT : NBL ratio above the 95 th percentile. The ratio is therefore a strong marker for trisomy 21. Copyright © 2011 ISUOG. Published by John Wiley & Sons, Ltd.