Background and objectives. The study aimed to compare the a fetoprotein (AFP), total beta-human chorionic gonadotropin (hCG), and unconjugated estriol (uE3) levels in the second-trimester triple screening for Down syndrome with different regions, and to analyse the related factors that influenced the screening performance. Materials and methods. The study was conducted between February 2007 and November 2010 in Beijing Tongren Hospital, P. R. China. Data derived from 7,647 normal singleton pregnancies between 14 and 21 weeks of gestation were examined. The regressed median values in different gestational ages were compared with earlier published data from other regions. The distribution of median values and multiples of median (MoM) of AFP, hCG and uE3 according to maternal age and weight in normal pregnancies were described. Statistic parameters for AFP, hCG, and uE3 (based on log(10) MoM values) were compared with earlier published data from other studies. Results. There were significantly increasing trends for AFP (p < 0.001) and uE3 (p < 0.001), and a significantly decreasing trend for hCG (p < 0.001) in the second trimester. There were significantly decreasing trends with increasing maternal weight for all the markers and their MoMs (p < 0.001). The distribution of the log(10) MoM marker values were Gaussian for the three parameters. Conclusions. Ethnic and laboratory variance should be taken into account in the second trimester triple screening for Down syndrome. The parameters of maternal serum markers should be calculated using local data, and the algorithm modified to match the screening achievable for the local population.