妊娠时若有18三体存在,母体血清中甲胎蛋白(MSAFP)、hCG和非结合性E_3的浓度一般低于正常,利用这些发现可以筛查中孕期的18三体。分析物的浓度以正常妊娠的中位值倍数(MoM)来表示。如MSAFP≤0.75 MoM,hCG≤0.60 MoM,E_3≤0.55 MoM,估计可测出60％的18三体胎儿,假阳性率为0.4％。这种固定标准法的缺陷在于评估风险时未考虑母龄与常染色体三倍体发生率之间的联系。另一种方法(风险法)在计算每个孕妇产生18 If trisomy 18 is present during pregnancy, concentrations of maternal serum alpha-fetoprotein (MSAFP), hCG, and unconjugated E_3 are generally lower than normal, making use of these findings to screen for trisomy 18 during pregnancy. Analyte concentrations are expressed as the median multiple of normal pregnancy (MoM). If MSAFP≤0.75 MoM, hCG≤0.60 MoM, E_3≤0.55 MoM, it is estimated that 60% of trisomy 18 fetuses have a false positive rate of 0.4%. The drawback of this fixed standard approach is that risk is not accounted for by the link between mother-age and autosomal triploidy. Another method (risk method) produces 18 for each pregnant woman