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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.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.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.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.

In recent years has been intensified an effort to develop early more exact non-invasive methods for prenatal diagnosis. We report our experience to screen trisomy 21 and other chromosomal abnormalities in the first trimester in so-called OSCAR (One-Stop Clinic for Assessment of Fetal Risk).It is a retrospective study held at the Department of Medical Genetic and Fetal Medicine, Teaching Hospital, Olomouc.A group of 2110 pregnant women has been screened by maternal age as a background risk, fetal nuchal translucency, nasal bone as ultrasound markers as well as maternal serum level of free beta-subunit human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) as a biochemical markers between 11 - 13+6 weeks of pregnancy from January 2004 to December 2005. The Fetal Medicine Foundation Guidelines and software were the methods and tools used for screening. Karyotyping by chorionic villous sampling or amniocentesis was offered to women with risksor = 1 in 300.We reported 100% sensitivity for this method for a 4, 6% false positive. There was detected 21 chromosomal abnormalities in which 10 were trisomy 21. This is the first result to be published for this screening method in the Czech Republic.In our experience first trimester screening for trisomy 21 and other aneuploidies has a high sensitivity with a low false positive rate and can be delivered in an efficient manner in a one-stop multidisciplinary clinic.