Objectives: The aim of this study is to evaluate the significance of nasal bone as a marker for trisomy 21 in the group of women that underwent invasive procedures in our center at 11 to 14 weeks’ gestation. Methods: The data of 181 women who had undergone the invasive procedures were evaluated for the presence or absence of nasal bone retrospectively and were correlated with fetal karyotype. Results: A successful view of the fetal profile was obtained in 135 fetuses. The nasal bone was absent in 5 of 8 fetuses with trisomy 21 and in 3 of 4 fetuses with trisomy 18. In the group of chromosomally normal fetuses the nasal bone was absent in 4 of the 123 cases. The false positive rate of our screening program dropped from 4.5% to 2.5% after the introduction of the nasal bone evaluation into our risk calculation model for trisomy 21. Conclusion: Nasal bone evaluation improved the detection of trisomy 21 in the first trimester in our screening program and reduced the need for invasive procedures in our department. Absence of the nasal bone showed a sensitivity of 63 % for a 5 % false positive rate for trisomy 21 in our study. It is rarely observed in chromosomally normal fetuses (2.5%). An appropriate training is mandatory in order to achieve acceptable results.

        Key words: first trimester screening, trisomy 21, nasal bone