Nasal bone length: prenasal thickness ratio: a strong 2D ultrasound marker for Down syndrome

ABSTRACT Objectives To evaluate the feasibility of incorporating two‐dimensional ultrasound measurements of nasal bone length (NBL) and prenasal thickness (PT) into the second‐trimester anomaly scan and to determine whether the NBL : PT ratio could help in differentiating euploid and Down syndrome fetuses. Method Two‐dimensional measurements of NBL and PT were obtained from the midsagittal plane of the fetal head at 14–28 weeks of gestation in a Caucasian population at risk for aneuploidy. The screening performances of NBL, PT, and the ratios NBL : PT and PT : NBL were analyzed in euploid (n = 1330) and Down syndrome (n = 33) fetuses. Results Nasal bone length and PT alone showed strong correlations with Down syndrome (sensitivity: 76% at 1.88% and 2.35% false positive rate, respectively). However, the NBL : PT ratio showed an even stronger correlation with Down syndrome (false positive rate: 0.9%, sensitivity: 97%). The mean NBL : PT ratio showed a gradual increase from 1.48 to 1.79 (a 21.2% increase) between 14 and 28 weeks of gestation. Conclusion Two‐dimensional ultrasound measurements of NBL and PT, particularly the NBL : PT ratio, are highly sensitive markers for Down syndrome fetuses. © 2014 The Authors. Prenatal Diagnosis published by John Wiley & Sons, Ltd. What's already known about this topic?Preliminary 2D and retrospective 3D studies show that the ratio of prenasal thickness to nasal bone length (PT : NBL) is a strong marker of second‐trimester Down syndrome fetuses. What does this study add?The ratio of nasal bone length to prenasal thickness (NBL : PT) had performed better than its inverse counterpart for the screening of trisomy 21.In euploid fetuses, the PT : NBL and the NBL : PT ratios showed gradual increases over time.Two‐dimensional ultrasound measurements of NBL and PT were successfully incorporated into the routine second‐trimester anomaly scan.