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

Abstract

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. © 2011 Nordic Federation of Societies of Obstetrics and Gynecology.