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

Abstract

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 secondtrimester 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. Current second-trimester screening for aneuploidy includes maternal serum screening and targeted ultrasound evaluation. A number of sonographic abnormalities or "soft markers" have been associated with trisomy 21; some authors have combined these results to provide a risk score or probability for trisomy 21. 3,4 The goal of such prenatal screening programs is accurate detection of fetal aneuploidy and avoidance of unnecessary invasive diagnostic testing. For this purpose, ultrasound has proved to be an invaluable diagnostic and screening tool. The absence of the fetal nasal bone on prenatal sonogra- Received December 6, 2004, from the Department of Obstetrics and Gynecology, Division of MaternalFetal Medicine of Perinatal Medicine (L.T.T., D.B.C., S.B.U., L.E.S.), and Department of

Absent or Shortened Nasal Bone Length and the Detection of Down Syndrome in Second-Trimester Fetuses

“…These studies have shown that some second trimester Down syndrome fetuses do have an apparent absence of the nasal bone whereas others show hypoplasia relative to unaffected fetuses. [2][3][4][5][6][7][8][9] Both the use of MoM and biparietal diameter/ nasal bone ratios seem to provide practical approaches to screening for fetal Down syndrome, because we were unable to demonstrate any significant difference between the two methods. The various studies that have used biparietal diameter/nasal bone ratios have reported widely different sensitivities and specificities for same cutoff (5-7, this study).…”

Uptake of invasive prenatal diagnostic tests in women after detection of soft markers for chromosomal abnormality on ultrasonographic evaluation

“…The risk in percent form is easy for the families to understand and interpret. Nuchal fold thickness, 3 nasal bone hypoplasia 4 and mild ventriculomegaly 5 in isolation have a higher risk of aneuploidy compared to other markers in isolation. The relative risk for nuchal fold thickening of >4, >5, >6 mm is 18, 28, 36 respectively.…”

Investigation of Bone Ratios for Prenatal Fetal Assessment in Taiwanese Population

“…A total of 136 fetuses were evaluated for the association between the BPD/NBL ratio and trisomy 21 in the second-trimester [19]. In many subsequent studies, assessments of the NBL/BPD ratio were done in the second trimester.…”

“…Reviewing several past studies, in addition to discussing the relationship between nasal bone length (NBL) and gestational age or biparietal diameter (BPD), the BPD/NBL ratio ≥ 10 and ≥ 11 were proposed for the definitions of nasal bone hypoplasia in the second trimester [13,[15][16][17][18][19].…”