More invasive procedures are done to detect each case of Down's syndrome in younger women

Objective. To evaluate the performance of first‐trimester combined screening in 5‐year periods according to maternal age in a low‐risk population. Design. A prospective study. Setting: Multicenter study in Finland. Population: A total of 76 949 voluntary women with singleton pregnancies participated in first‐trimester combined screening in public healthcare between 1 May 2002 and 31 December 2008. Methods. The serum samples were analyzed using the PerkinElmer AutoDELFIA® time‐resolved fluoroimmunoassay kit for the measurement of pregnancy‐associated plasma protein‐A and free beta‐human chorionic gonadotropin. Nuchal translucency was measured by trained personnel (midwives or physicians) in a university or central hospital. Main Outcome Measures. Performance, detection rate, false positive rate and the number of invasive procedures needed to detect a single case of Down's syndrome were analyzed. Results. There was a direct connection between maternal age and the prevalence of Down's syndrome with a low prevalence in young women being 1:1 193 in the 25–29 age group and 1:150 in the 35–39 age group. Consequently, for a fixed false positive rate of 5%, the number of invasive procedures needed to detect one case of Down's syndrome is higher in younger women to achieve the same detection rate. Conclusions: In combined first trimester screening the risk for Down's syndrome is individual, varying with maternal age. This should be taken into consideration when counseling women.