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Second-Trimester Biparietal Diameter/Nasal Bone Length Ratio Is an Independent Predictor of Trisomy 21

Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine of Perinatal Medicine (L.T.T., D.B.C., S.B.U., L.E.S.), and Department of Radiology (L.M.M.), University of Washington, Seattle, Washington USA.

Address correspondence to Lan T. Tran, MD, Box 356460, University of Washington, Seattle, WA 98195 USA. E-mail: lant2{at}u.washington.edu

Objective. The purpose of this study was to evaluate the association between the second-trimester fetal biparietal diameter/nasal bone length (BPD/NBL) ratio and trisomy 21. Methods. Thirty-one cases of trisomy 21 for which complete ultrasound images included the nasal bone were identified from the University of Washington prenatal diagnosis database and matched to 136 euploid fetuses based on maternal age, indication for referral, and gestational age. Results. The mean NBL was shorter (mean ± SD, 2.3 ± 1.7 mm versus 3.9 ± 1.2 mm; P < .001) and the BPD/NBL ratio was greater (17.7 [range, 6.2–114] versus 11.7 [range, 5.8–80]; P < .001) in the fetuses with trisomy 21. The risk of trisomy 21 increased 2.4-fold (95% confidence interval [CI], 1.7–3.4) with every 1-mm decrease in NBL and increased 1.08-fold (95% CI, 1.03–1.12) with each unit increase in the BPD/NBL ratio (P < .001). A multiple logistic regression model was constructed and included the BPD/NBL ratio, maternal indications (age 35 years, positive serum screening results, or both, yielding a risk of <1 per 270 for trisomy 21), and sonographic markers as covariates. The BPD/NBL ratio was found to be an independent predictor of trisomy 21 (odds ratio, 1.08; 95% CI, 1.03–1.11). An analysis of receiver operating characteristic curves revealed an improvement after the BPD/NBL ratio was added to a model containing the current second-trimester screening based on maternal age, serum screening, and sonographic markers (receiver operating characteristic curve area, mean ± SE, 0.89 ± 0.03 for the model with the BPD/NBL ratio versus 0.76 ± 0.06 without the BPD/NBL ratio; P = .009). Conclusions. The second-trimester BPD/NBL ratio was a significant and independent predictor of trisomy 21. An assessment of the BPD/NBL ratio may improve the diagnosis of trisomy 21 when used with current prenatal screening practices.

Key Words: nasal bone length • screening • trisomy 21 • ultrasound

Abbreviations: BPD, biparietal diameter • CI, confidence interval • NBL, nasal bone length • ROC, receiver operating characteristic

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