Abstract: Abstract Objective: To establish the medians of maternal serum alpha-fetoprotein (AFP) and free human chorionic gonadotropin (Free β-hCG) for second trimester Down syndrome screening in Tianjin district, in order to discuss its clinical medical value. Method: We measured serum level of AFP and Free β-hCG from 100,543 pregnancies ranged from 14 weeks to 20+6 weeks using time-resolved fluoroimmunoassay. The Medians Tool of the PE risk assessment software（LC3.0） was used to determine 61,881 pregnancies’ serum AFP and free β-hCG medians value, modified by median coefficients，and established the Weight equation. 38,662 pregnancies’ serum AFP and free β-hCG levels were adjusted by median shift to establish median values system for second-trimester Down syndrome screening .Other 13,001 pregnancies’ serum AFP and free β-hCG levels were measured. The screening results were calculated using the standard algorithm based on the Caucasian median value system and the Tianjin median value system. Patients with an increased risk of Down syndrome pregnancy (≥270) were offered genetic amniocentesis. Result: Medians from 13,001 pregnancies matched the established median formula. The Median factor of AFP and free β-hCG were 1.01 and 1.00respectively. The detection rate （DR）of the Tianjin median value system for second-trimester Down syndrome screening was 77.78%, false positive rate（FDR） was 5.33%, positive predictive value was 10.00‰,and the DR based on the Caucasian median value system was 77.78%, the FDR was 8.30%, positive predictive value was 6.49‰. Conclusion: Ethnic variance exists between the Caucasian and Tianjin populations. The accuracy of screening is increased by the use of race-specific medians.

Key words: Median value, second-trimester serum screening, Down syndrome