IntroductionFetal nasal bone hypoplasia has been proven to be a strong marker for trisomy 21 during second trimester ultrasonography. Rather than evaluating the nasal bone hypoplasia according to the nomograms, it may be possible to evaluate it with a more practical and specific method.Material and methodsRetrospectively collected data were provided from the saved and stored patient data between 2012 and 2017. Nasal bone length and nasal tip length were measured and the ratio was calculated. The ratio ≤ 1/3 was defined as the threshold ratio for the definition of the marker. Nasal bone nasal tip length (NB/NL) ratio values were compared among groups with trisomy 21 and euploid fetuses.ResultsA total of 59 fetuses had trisomy 21 and 2304 euploid fetuses were included in the study. The ratio of nasal bone to nasal tip length was 1/3 and below in 21/59 (35.5%) of fetuses with trisomy 21 whereas this ratio was 31/2304 (1.3%) in euploid fetuses. The difference was statistically significant (p < 0.001). Sensitivity, specificity, positive likelihood ratio and negative likelihood ratio of nasal bone/nasal tip ratio ≤ 1/3 were 35.5%, 98.6%, 26.4 and 0.6 for Down syndrome, respectively.ConclusionsThis study suggested that the nasal bone to nasal tip length ratio may be a new promising and practical sonographic marker in the second trimester genetic sonography. Although its sensitivity is not high, it seems to be more practical and specific than the evaluation of fetal nasal bone length.