In the serum biochemical screening group, in order to achieve OAPR higher than that of intermediate-risk and obtain a higher detection rate of trisomy 21, the cut-off value of abnormal biomarker was defined as free β-hCG MoM ≥ 2.75 or PAPP-A MoM ≤ 0.5 (Table 4). The gene discussed is PAPPA; the disease is trisomy 21.