Fetal nasal bone length: reference range and clinical application in ultrasound screening for trisomy 21.

OBJECTIVES

Fetuses with trisomy 21 typically present with subtle facial abnormalities, including a hypoplastic nasal bone. The aim of this study was to provide a reference range for the length of the fetal nasal bone and to test its value in second-trimester ultrasound screening for trisomy 21.

DESIGN

A reference range of fetal nasal bone length was established from cross-sectional data on 1923 consecutive singleton pregnancies scanned at 16-24 weeks' gestation in women older than 35 years. Screening for trisomy 21 was prospectively studied using the measurement of fetal nasal bone lengths smaller than the 5(th) percentile as a cut-off value.

RESULTS

Follow-up was possible in 1631 cases (84.8%). Trisomy 21 was found in 22 cases (1.35%). Nasal bone length measurement increased as a function of gestational age (P < 0.05) showing a linear relationship. Screening for trisomy 21 using the 5(th) percentile as a cut-off value resulted in a sensitivity of 59.1% for a 5.1% false-positive rate. The likelihood ratio was 11.6.

CONCLUSION

Screening for trisomy 21 using fetal nasal bone length measurements showed a sensitivity comparable to that of maternal biochemistry for a given false-positive rate of 5%. Association of nasal bone lengths with other sonographic markers, taking into account the background risk for maternal and gestational age, may further improve sensitivity and reduce false positives, allowing avoidance of unnecessary invasive diagnostic procedures.