Abstract: Objective To investigate the clinical value of second trimester maternal serum screening for Down's syndrome(DS),18-trisomy syndrome,and neural tube defects(NTD).Methods The serum markers(alpha-fetoprotein and beta human chroionic gonadotrophin) were detected in 120 309 pregnant women at second trimester(15-21 weeks) with time-resolved fluorescence immunoassay(DELFIA).The detection data were analyzed with Lifecycle 3.0 software.Results There were 5 022 pregnant women identified as at high-risk,with a positive rate of 4.17%.Amniotic fluid or umbilical cord blood puncture was conducted among 79.51% of the women with high risk of DS and 18-trisomy syndrome(3 573/4 494).There were 49(1.37%) cases of abnormal fetal karyotype, 22 cases of DS,6 cases of 18-trisomy syndrome,and 21 cases of other abnormal karyotype.In high-risk pregnant women who were younger or older than 35 years,the abnormal karyotype detection rate was 1.37%.Conclusion Second trimester maternal serum markers(AFP and β-HCG) detection is efficient in prenatal screening for DS,18-trisomy syndrome,and NTD.