Interobserver Variability of the Measurement of Fetal Nasal Bone Length between 11+0 and 13+6 Gestation Weeks among Experienced and inexperienced Sonographers

Purpose: Examination of fetal nasal bone (NB) by ultrasound between 11+0 and 13+6 gestation weeks has been proposed as an additional tool in the detection of trisomy 21 and therefore its application and implementation are used in a broad range. The study aimed at evaluating the interobserver feasibility of the measurement of fetal nasal bone length in comparison with experienced and inexperienced monographers. Materials and Methods: The study population was comprised of women who chose to have first trimester screening (FTS) at the Fetal Medicine Unit of the University Medical School of Hannover. Two experienced (> 400 FTS examinations, sonographer 1 and 2) and one inexperienced sonographer (95 FTS examinations, sonographer 3) were asked to measure the nasal bone length consecutively and independently of each other. Statistical analysis was performed for any differences and variations in the results. Results: The fetal profile was examined in 220 cases. The median nasal bone length by sonographer one was 2.4cm, sonographer two 2.4cm and sonographer three 2 cm. The differences between the results of sonographer I and 3 as well 2 and 3 were statistically significant. There were no significant variations between the results of sonographer 1 and 2. There was also no significant difference in the results concerning nuchal translucency and crown-rump length among the three examiners. Conclusion: The uncertainty and the difficulties of an inexperienced examiner with the presenting of the nasal bone, as shown by published data sets as well as by the variability of the measurement results of this study, with all the consequences in the risk calculation and counseling show that this tool should only be implemented by experienced and quality-controlled sonographers with a minimum amount of examinations. Because of its major impact in risk calculation and the importance of the nasal bone as a sonographic marker, documentation of the sonographer's skills is mandatory for the use of the nasal bones as an additional sonographic marker in first trimester screening.