Objective To study the method of prenatal screening Down's syndrome in China,and to establish the median of each gestational week in the population of the region.Methods Auto time resolved fluoroimmunoassay system was used to detect the maternal serum markers in 7501 pregnant women at about 9~20 weeks of gestation-pregnancy associated plasma protein A(PAPP A)?alpha fetoprotein(AFP)?free beta human chorionic gonadotropin(free β HCG),and the median was calculated with weighted nonlinear regression.Res...