Measurement of nasal bone length at 11-14 weeks of pregnancy and its potential role in Down syndrome risk assessment.

OBJECTIVES: To assess the feasibility of measuring nasal bone length in first-trimester pregnancy and to confirm if the absence of a fetal nasal bone is a marker for Down syndrome. METHODS: Fetal nasal bone assessment was attempted in 1089 consecutive singleton pregnancies between 11 and 14 weeks' gestation. All ultrasound examinations were performed transabdominally in three separate centers. If the nasal bone was present, nasal bone length was measured. RESULTS: Nasal bone assessment was successfully achieved in 1027 of 1089 (94.3%) ultrasound examinations. Within this group nasal bone was absent in 10 of 1000 (1.0%) unaffected cases, 10 of 15 (66.7%) Down syndrome cases and 5 of 12 (41.7%) cases with other pathological conditions. Regression analysis showed a significant increase (P < 0.0001) in nasal bone length from 2.48 mm at a crown-rump length of 45 mm to 3.12 mm at a crown-rump length of 84 mm. The nasal bone length in the five cases of Down syndrome in which the nasal bone was present was less than the median measurement of unaffected cases. Using modeling, the combination of nasal bone with maternal age, nuchal translucency, free beta-human chorionic gonadotropin (hCG) and pregnancy associated plasma protein-A (PAPP-A) achieved a detection rate of 95% with a false-positive rate of 2.9%. At a fixed 1% false-positive rate, the detection rate was 91%. CONCLUSIONS: Absence of the nasal bone can be used as a marker for Down syndrome in the first trimester of pregnancy. Inclusion of the nasal bone in the current first-trimester screening protocol along with nuchal translucency, free beta-hCG and PAPP-A can achieve high detection at a very low false-positive rate. Large datasets are needed to confirm whether the measurement of nasal bone length provides additional benefits beyond the assessment of the presence or absence of the nasal bone.