Abstract： Objective To establish the reference value of inhibin-A in the serum of pregnant women with gestational age from 15 to 20+6 weeks in Guangdong province,and assess the efficiency of inhibin-A and quadruple test in Down syndrome screening. Methods A total of 2802 singleton pregnancies receiving triple test screening in Guangzhou Women and Children's Medical Center from March 2008 to December 2010 were included in this study.Inhibin-A was measured by automatic enzyme-linked immunosorbent chemiluminescence assay. The concentration of inhibin-A was expressed as multiples of the median (MoM),and adjusted with maternal weight and gestational age.Parameters of SURUSS were used to recalculate the risk of Down syndrome.The efficacy of single marker and combination were evaluated by receiver operating characteristic curve and the area under the curve. Results (1) In normal singleton pregnancies,the median concentration of inhibin-A was 286.60,267.10,249.10,243.40,242.30 and 256.60 pg/ml respectively for each week of gestational age from 15 to 20+6 weeks.The distribution of inhibin-A in each gestational week was relatively stable.The mean concentration [(852.83±370.04) pg/ml] and MoM (2.82) of inhibin-A in twelve pregnant mothers with Down syndrome fetuses were significantly higher than those without [(293.28±149.46) pg/ml (t=5.37,P＜0.05) and 1,respectively].(2) The detection rate was 83.3％ (10/12) by using the quadruple test including free human chorionic gonadotropin-β,alphafetoprotein,unconjugated estriol and inhibin-A at false positive rate of 5.8％; while when the detection rate of triple test including alpha-fetoprotein,free human chorionic gonadotropin-β and unconjugated estriol was 83.3％,the false positive rate was 7.7％.When the false positive rate was set to 5.0％,the area under the curve of inhibin-A,alpha-fetoprotein,free human chorionic gonadotropin-β and unconjugated estriol was 63.7％,20.5％,46.1％ and 4.8％,respectively,and the relative area under the curve of routine triple test and quadruple test was 45.5％ and 63.1％,respectively. Conclusions Inhibin-A is suggested to be the most effective marker used for secondtrimester screening,which could be used for second trimester Down syndrome screening in Chinese population combined with existing three markers.