Improved contingent screening strategy increased trisomy 21 detection rate in the second trimester.

Purpose: This study aimed to establish suitable threshold values for biochemical indicators in low-risk pregnant women who underwent second trimester screening and design strategies for consecutive prenatal testing to increase trisomy 21 detection.

Methods: This study examined singleton pregnant women who underwent double, triple, or quadruple screening in the second trimester over six years. To obtain adequate detection efficiency for low-risk pregnancies, threshold values for serum biochemical indicators were established, and a cost-effectiveness assessment of the improved contingent screening strategy was conducted.

Results: Participants were included in serum double- (n = 88,550), triple- (n = 29,991), and quadruple-screening (n = 15,004) groups. Threshold values were defined as having a free beta subunit of human chorionic gonadotropin (free β-hCG) multiple of the median (MoM) ≥ 2.50, alpha-fetoprotein (AFP) MoM ≤ 0.50, or unconjugated estriol (uE3) MoM ≤ 0.70 for low risk. Low-risk pregnancies, comprising 1.35% (988/73,183), 4.45% (1,171/26,286), and 11.91% (1,559/13,085) of the double-, triple-, and quadruple-screening groups, respectively, underwent further non-invasive prenatal screening. In the double-, triple-, and quadruple-screening groups, we detected 11.76% (2/17), 40.00% (2/5), and 66.67% (2/3) of trisomy 21 cases with false negative results, respectively, with the overall detection rates of 85.00% (85/100), 90.63% (29/32), and 95.24% (20/21), respectively, and decreased ratio of overall costs of 5.26%, 16.63%, and 24.36%, respectively.

Conclusion: Utilizing threshold values of AFP, free β-hCG, and uE3 to trigger further non-invasive prenatal screening may increase trisomy 21 detection in pregnancies deemed low risk in the second trimester while reducing the overall costs of screening strategies.