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Background <p>This study aimed to evaluate the diagnostic performance of fetal nasal bone length (NBL), the biparietal diameter to nasal bone length ratio (BPD/NBL), and nasal bone length multiples of the median (NBL-MoM) values for Down syndrome (DS) screening between 17 and 25 weeks of gestation in a population-specific cohort.</p> Methods <p>A retrospective analysis was conducted on fetuses between 17 and 25 gestational weeks. Normative data for NBL and NBL-MoM were generated using gestational age–specific reference medians, and regression equations were established. The NBL, BPD/NBL ratio, and NBL-MoM values were compared between DS and euploid groups. NBL-MoM values were calculated using raw (non-transformed) measurements divided by the expected gestational age–specific median, and euploid status was confirmed by normal postnatal outcomes. Sensitivity, specificity, and likelihood ratios were calculated for various cutoff points.</p> Results <p>NBL showed a significant linear correlation with gestational week (R² = 0.31, <i>p</i> &lt; 0.001), while BPD/NBL was independent of gestational age. In DS cases, absence of the nasal bone was observed in 40% of fetuses, compared to 0.03% in controls (sensitivity: 40%, specificity: 100%, LR⁺: 1228). An NBL &lt; 2.5&#xa0;mm yielded a sensitivity of 49% and specificity of 99.9% (LR⁺: 746). Using the 5th percentile as a cutoff, sensitivity and specificity reached 86% and 98.9%, respectively (LR⁺: 75, LR⁻: 0.15). ABPD/NBL ratio ≥ 9 yielded 93% sensitivity and 97.5% specificity (LR⁺: 37). NBL-MoM values &lt; 0.65 yielded 97% sensitivity and 98% specificity.</p> Conclusion <p>Our findings support the use of population-specific reference ranges for fetal NBL, BPD/NBL ratio, and NBL-MoM values in the second trimester to improve the screening accuracy for Down syndrome. These sonographic markers—particularly NBL &lt; 5th percentile, BPD/NBL ≥ 9, and NBL-MoM &lt; 0.65—may provide valuable, non-invasive indicators to support clinical assessment.</p> Trial registration <p>This study was retrospectively registered and authorized by the local Ethics Committee of Ankara University (approval number: 2025/696; date: 03 September 2025).</p>

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Predicting down syndrome: a comparative evaluation of nasal bone length in mid-trimester pregnancy

  • 期刊论文

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  • 发表日期：

    2026年3月11日

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  • Mustafa Koçar,
  • Hasan Süt,
  • Coşkun Ümit,
  • Erdal Şeker,
  • Gülşah Aynaoğlu Yıldız

摘要

Background

This study aimed to evaluate the diagnostic performance of fetal nasal bone length (NBL), the biparietal diameter to nasal bone length ratio (BPD/NBL), and nasal bone length multiples of the median (NBL-MoM) values for Down syndrome (DS) screening between 17 and 25 weeks of gestation in a population-specific cohort.

Methods

A retrospective analysis was conducted on fetuses between 17 and 25 gestational weeks. Normative data for NBL and NBL-MoM were generated using gestational age–specific reference medians, and regression equations were established. The NBL, BPD/NBL ratio, and NBL-MoM values were compared between DS and euploid groups. NBL-MoM values were calculated using raw (non-transformed) measurements divided by the expected gestational age–specific median, and euploid status was confirmed by normal postnatal outcomes. Sensitivity, specificity, and likelihood ratios were calculated for various cutoff points.

Results

NBL showed a significant linear correlation with gestational week (R² = 0.31, p < 0.001), while BPD/NBL was independent of gestational age. In DS cases, absence of the nasal bone was observed in 40% of fetuses, compared to 0.03% in controls (sensitivity: 40%, specificity: 100%, LR⁺: 1228). An NBL < 2.5 mm yielded a sensitivity of 49% and specificity of 99.9% (LR⁺: 746). Using the 5th percentile as a cutoff, sensitivity and specificity reached 86% and 98.9%, respectively (LR⁺: 75, LR⁻: 0.15). ABPD/NBL ratio ≥ 9 yielded 93% sensitivity and 97.5% specificity (LR⁺: 37). NBL-MoM values < 0.65 yielded 97% sensitivity and 98% specificity.

Conclusion

Our findings support the use of population-specific reference ranges for fetal NBL, BPD/NBL ratio, and NBL-MoM values in the second trimester to improve the screening accuracy for Down syndrome. These sonographic markers—particularly NBL < 5th percentile, BPD/NBL ≥ 9, and NBL-MoM < 0.65—may provide valuable, non-invasive indicators to support clinical assessment.

Trial registration

This study was retrospectively registered and authorized by the local Ethics Committee of Ankara University (approval number: 2025/696; date: 03 September 2025).