In order to establish normative median values of maternal serum total human chorionic gonadotropin (hCG) at our own laboratory, 2711 normal sera were collected from uncomplicated, singleton Chinese pregnant women, including 1705 samples measured by radioimmunoassay (RIA) between 13 and 24 weeks' gestation and 1006 samples measured by enzyme immunoassay (EIA) between 14 and 20 weeks' gestation. The hCG secretion pattern throughout midtrimester measured by RIA is similar to that of measured by both RIA and EIA have a steep drop initially and then steadily decline from 17 weeks of gestation. The total (hCG levels measured by RIA and EIA showed different normative median values and distribution. If it was measured by RIA between 14 and 20 week's gestation, there were 3.7% and 17.8% of pregnancies with serum total hCG levels less than 0.25 multiple of the median (MoM) and 0.5 MoM, retrospectively, and 11.6% and 5.5% among them with serum levels above 2.0 MoM and 2.5 MoM, respectively. When it was measured by EIA, there were 0.7% and 9.2% of pregnancies with serum total hCG levels less than 0.25 MoM and 0.5 MoM, respectively, and 8.9% and 3.7% among them with serum levels above 2.0 MoM and 2.5 MoM, respectively. Accurate and satisfactory interpretation of maternal serum screening for Down syndrome depends on establishment of a well-developed normative median value for each week of gestation. Any laboratory intends to provide hCG for maternal serum screening should have its own reference data by its own immunoassay method.