Nuchal translucency and nasal bone for trisomy 21 screening: single center experience.

Aim: To evaluate the feasibility and diagnostic accuracy of fetal nuchal translucency and nasal bone assessment at 11-14 weeks for screening of trisomy 21 at a single center.

Methods: Nuchal translucency measurement and nasal bone evaluation in relation to the fetal karyotype for singleton fetuses were retrospectively assessed at the Ospedale Microcitemico, Cagliari, Italy, in a three-year period (2001-2004). Nuchal translucency was considered enlarged if greater than or equal to the 95th centile for crown-rump length (CRL) of the reference ranges, and nasal bone was described as present or absent during the evaluation of the fetal facial profile. Sensitivity and specificity for trisomy 21 were assessed for nuchal translucency and absent nasal bone.

Results: Among 32,000 cases recorded in the database including fetuses from 11 to 14 weeks, 16,654 fetuses were included in the study with both nuchal translucency measurement and nasal bone evaluation. Median maternal age was 32 years (range, 14-49). In 854 fetuses (5.1%), nuchal translucency was greater than the 95th centile, and 744 (87.1%) of them had a normal karyotype. Among 141 (0.8%) diagnosed cases of chromosomopathies, there were 96 cases of trisomy 21. Nuchal translucency was enlarged in 110 chromosomopathies and in 72 trisomies 21. Sensitivity was 75.0% (95% confidence interval [CI], 65.5-82.6), and specificity 95.5% (95% CI, 95.2-95.8). In fetuses with enlarged nuchal translucency and normal karyotype, there were 30 structural defects (4%), and among these, 15 heart defects (2%). Measurement of nuchal translucency was possible in all cases where it was attempted. In 13 cases (0.1%), it was not possible to determine the visibility of the nasal bone. In 16,486 cases, the nasal bone was defined as visible and in 155 cases (0.9%) the nasal bone was described as absent. The nasal bone was absent in 56 trisomies 21 and in 23 other chromosomopathies, as well as in 76 normal karyotype fetuses. The sensitivity was 58.3% (95% CI, 48.3-67.7) and specificity 99.5% (95% CI, 99.4-99.6). The sensitivity of enlarged nuchal translucency and nasal bone was 80.2% (95% CI, 71.1-86.9).

Conclusions: Enlarged nuchal translucency and absent nasal bone are useful markers of trisomy 21 in the first trimester ultrasound screening, increasing the sensitivity of detection of affected fetuses.