Down syndrome maternal serum marker screening after 18 weeks' gestation.

Women having access to prenatal care late in pregnancy may still wish to benefit from maternal serum screening for Down syndrome. Therefore, we established reference values for alpha-feto protein (AFP) and free beta-human chorionic gonadotrophin (beta-hCG), and assessed the diagnostic value of maternal serum marker screening at 18-35 weeks' gestation based upon a series of 4072 sera from unaffected pregnancies and 118 sera from pregnant women with fetuses affected by Down syndrome. Using a 1/250 risk cut-off, a detection rate of 72.9% (95% CI = 71.5-74.3%) was achieved with a false-positive rate of 7.51% (95% CI = 6.71-8.3%). This was not significantly different from the percentages observed in our 14-17 weeks routine screening (50 596 patients): 71.9% (95% CI = 71.5-72.3%) and 6.48% (95% CI = 6.28-6.68%), respectively. Detection and screen-positive rates were, respectively, 51.3% (95% CI = 35.6-67.0%) and 5.95% (95% CI = 5.12-6.68%) in women aunder 35 years of age, and 84.8% (95% CI = 76.9-92.7%) and 24% (95% CI = 20.7-27.3%) in women aged 35 years and over. In conclusion, maternal serum marker screening is feasible at 18 weeks' gestation and later, which may be of interest in selected cases.