Abstract: Objective：To investigate the performance of first trimster screening for aneuploidies in the combined test of maternal age，fetal translucency thickness and serum free β-human chorionic gonadotropin （β-hCG） and pregnancy- associated plasma protein-A（PAPP-A）. Methods：Screening with the combined test at 11 to 13+6 weeks of gestation was performed in singleton pregnancies，including 497 euploid fetues and 3 trisomy 21syndrome. The blood test and ultrasound scan were carried out in the same visit. The measure free β-hCG and PAPP-A were converted into a multiple of the median （MoM） for gestational age，adjusted for maternal weight，ethnicity，smoking status，method of conception. Results：① 500 cases of pregnant women in early screening of trisomy 21 and 6 high-risk people，of which 3 were confirmed by prenatal diagnosis. Another 40 were age high-risk，5 of whom were normal. ② NT thickness count on the CRL. ③Serum PAPP-A levels ascend and free β-hCG of the maternal unaffected decrease with pregnant weeks increasing. Conclusions：①The high risk of ages can not be a rule in invasive prenatal diagnosis. ②The NT dependant on the CRL，and we should perform multicenter study to find the NT standard of chinese pepole. ③NT screening should be measured in a qualified hospital，and preferably 12 weeks of gestation.④The maternal serum level of PAPP-A and free β-hCG relate with the pregnant weeks.

Key words: Down syndrome, Multiphasic screening, Pregnancy trimester, first, Ultrasonography