To evaluate the performance characteristics of second trimester double-marker test for the detection of neonates with Down's syndrome.

From January 2006 to November 2008, 11 003 cases, between 14 and 20^{+ 6} weeks of pregnancy, who received screening for Down's syndrome in the department of Obstetrics and Gynecology, First Hospital of Beijing University were retrospectively analyzed. They were divided into older group (maternal age ≥35 years old, n=602) and younger group (maternal age <35 years old, n=10 401). Informed consent was obtained from all participates. The combination test of α-fetoprotein (AFP) and free β-subunit of human chorionic gonadotrophin (β-HCG) was used as the serum markers in this study. The results were calculated and analyzed with the software to screen the high-risk population of Down's syndrome, who were offered genetic counseling and the option of amniocentesis and cordocentesis.

① Of the 11 003 cases, 831 pregnant women with high-risk of Down's syndrome were included in high-risk group. 6 neonates were diagnosed as Down's syndrome, and 3 of them were confirmed as Down's syndrome. Among these 3 cases, 2 had abnormal ultrasound findings. ② At a cut-off value of 1 in 380 of Down's syndrome, the high-risk pregnancy with Down's syndrome was 831 cases, the detection rate (DR) was 66.7% (6/9), false positive rate (FPR) was 7.5% (829/10 998), and positive predictive value (PPV) was 0.72% (6/831). If using a cut-off of 1 in 270, the high-risk pregnancy with Down's syndrome was 534 cases, the detection rate was66.7%(6/9), false positive rate was 4.8%(527/10 998), and positive predictive value was 1.12% (6/534). The detection rate and positive predictive value of older group were similar with younger group. However, the false positive rate was little higher (27.45% vs. 6.39%) of older group. ③The safety of the amniocentesis procedure: the procedure-related loss rate was 1 in 458.

All pregnant women should be offered all the information needed before they get the screening test. The screening test is effective in older group. Ultrasound examination is useful in the screening strategy.