Objective To investigate the effect of different nuchal translucency thickness（NT） thickness medians on screening efficacy for Down’s syndrome in first-trimester.Methods Eleven NT credentialed sonographers of the prenatal diagnosis center in our hospital were picked out

who conducted NT scans on 23 446 cases of singleton pregnancies in firsttrimester.NT measurements were firstly transformed into multiply of NT median（NT-Mom） by the expected medians from the recommendation of Fetal Medicine Foundation（FMF） in order to survey whether the median NT-Mom measured by sonographers was in the range of 0.9～1.1.The sonographers were grouped three categories

including <0.9 undermeasuring group

0.9～1.1 incontrol group

and>1.1 overmeasuring group in terms of their medians.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 investigated and their performance of NT screening for Down’s syndrome were also evaluated.Furthermore

out of control rate of ultrasound practitioner were calculated by exponentially weighted moving average（EWMA） chart for NT quality.Multiple Logistic regression was applied to analyze the correlation between the false positive rate of FMF median and its related influencing factors.Results In 23 446 fetuses

NT median was 3.40 mm for 57 fetuses affected by trisomy 21 syndrome

5.40 mm for 24 fetuses affected by trisomy 18 syndrome

2.00 mm for 6 fetuses affected by trisomy 13 syndrome

5.60 mm for 23 fetuses affected by Turner’s syndrome

2.25 mm for 10 fetuses affected by other chromosome abnormalities

respectively.Among 11 sonographers

there were 2 sonographers classified as undermeasuring group

6 as in-control group

3 as overmeasuring group.NT-Mom medians in both undermeasruing group and overmeasuring groups after using local center-specific NT medians still stayed in the original interval

whereas their NT-Mom medians transformed with practitioner-specific NT medians returned into 0.9～1.1.The detection rates of Down’s syndrome using FMF

local central-specific and practitioner-specific NT median were77.19%

77.19% and 78.95%

respectively

with no statistically significant difference at a risk cutoff value as 1:250

but the false positive rate were 6.58%

6.23%

6.43%

which were significantly different（P<0.05）.Multiple Logistic regression analysis demonstrated that with a risk cutoff value as 1:250

the false positive rate of FMF median was associated with practitionerspecific NT medians

out of control rate of EWMA

and gestational age and OR were 453.7（95%CI:108.05～1922.97）

1.39（95%CI:1.10～1.75） and 1.30（95%CI:1.28～1.31）

respectively.Conclusion Utilization of practitioner-specific NT medians to a certain extent reduces variation of NT value and improves the screening performance for Down’s syndrome.Maintaining the appropriate performance of NT screening requires continuous supervision of NT measurement quality from sonographers.