Objective：To analyze results of prenatal screening for Down’ syndrome in the second trimester of pregnancy in 5 539 cases in the First Hospital of Lanzhou University from 2009 to 2012 and to identify the possible risk factors for high risk pregnant women. Methods：Serum alpha-fetoprotein and free-β human chorionic gonadotropin in pregnant women were detected based on the principal of time-resolved immunoflourometric assay. Risk of fetal Down’ syndrome was calculated by computer software taking into con－siderations of maternal age，gestational age，weight of pregnant women. Results：Among the 5 539 pregnant women，238 cases