Objective: to investigate the effect of using different NT expected medians on down screening performance Method: 23446 cases of singleton pregnancies were unselected for nuchal translucency scans in first trimester in the prenatal diagnose center of the third hospital affiliated Gang Zhou medical university between April 2013 and December 2016, of whom there were 57 twenty on trisomies and 61 other chromosomal abnormalities and 23328 fetuses with normal phenotype or karyotypes. All of those NT examinations were performed by 11 credentialed sonographers.NT measurements were transformed into NT-MoM (multiply of median) by the expected medians from the recommendation of FMF (Fetal Medicine Foundation). Based on the statistic analysis of the medians of NT-MoM from those 11 sonographers, according to the different individual interval NT-MoM were grouped three categories(<0.9，0.9~1.1,>1.1) . Then, local center-specific or practitioner-specific medians were used to convert NT measurements into NT-MoM. The changes of NT-MoM distributions from the three different categories were analyzed and the accessment of their performance of NT screening for Down syndrome were also conducted. Further more, out of control points percentages of ultrasound practitioner were analyzed with EWMA (exponentially weighted moving average) chart for nuchal translucency quality. Result: There were two ultrasound practitioners whose medians of NT-MoM calculated with expected NT medians from FMF recommendation were less than 0.9, six practitioners between 0.9 and 1.1, three practitioners more than 1.1. NT-MoM medians of the five ultrasound practitioners below 0.9 or above 1.1 were still below 0.9 or above 1.1 after using local center-specific expected NT medians, which category interval did not change, whereas their NT-MoM medians transformed with practitioner-specific NT medians returned into 0.9~1.1 range. Application of the three different NT medians resulted in no significant differences of 1:250 cutoff detection rate of down screening, but the false positive rate of 1:250 cutoff from the practitioner specific medians was lower than from the others. There was association between false positive rate and practitioner NT-MoM medians and the percentage of out control points on EWMA chart with multivariable logistic regression, which OR were 408.8（95%CI 260184.6~0.64，P<0.05）and 1.38（95%CI 1.13~1.69，P<0.05）respectively. In addition, the practitioner-specific group had lower percentage of out control points than others groups. Conclusion: Utilization of practitioner-specific NT expected medians reduces variation of NT measurement value to a certain extent to improve the performance of NT down screening. Continuous monitoring of sonographic practitioner NT measurement quality is needed to maintain proper NT down screening.