Objective: To investigate the ratio of prenasal thickness (PT) to nasal bone length (NBL) in normal and trisomy 21 fetuses in the second and third trimester in Chinese population. Methods: The NBL and PT were measured blindly by using 3D volumes in 143 normal fetuses and 31 trisomy 21 fetuses. Results: The mean PT (r = 0.83, p = 0.004) and NBL (r = 0.87, p = 0.0062) both increased with the gestation age, while the PT/NBL ratio (r = 0.12, p>0.10) remained stable. There was significant difference between normal and trisomy 21 fetuses (p<0.001). If we took the 95th of the normal fetuses as the cut-off value, the detection rate was only 46%. By using ROC curve to evaluate the screening value of PT/NBL ratio, the area under receiver operating characteristic (ROC) curve was 0.88 (95% confidence interval 0.81 to 0.94, p<0.0001). Conclusions: In Chinese population, the PT/NBL ratio is not a very strong ultrasound marker to predict trisomy 21 fetuses. However, it can be used as an ultrasound marker for Down syndrome screening during the second and third trimester of pregnancy.
