Objectives and Methods: To investigate the impact of prenatal diagnosis on trisomy 2 1 live births, we collected all prenatal and postnatal trisomy 2 1 cases (n = 1096) in the eastern half of Switzerland for the years 1980-1996. Results: Despite increasing prenatal detection rates of trisomy 21 foetuses (an increase of 169% in the last 5 versus the first 5 years of the study period) and subsequent termination of pregnancies, the number of liveborn Down syndrome children remained constant. The reason is a shift towards a higher mean maternal age from 28 to 30 years between 1980 and 1996. If mean maternal age at delivery was considered, the observed increase of trisomy 21 conceptions matched well with the calculated figures. Conclusion: If the tendency to have pregnancies at a more advanced age continues and if the use of prenatal diagnosis does not increase, an increase in incidence of Down syndrome liveborns may be expected in the first decades of the 21(st) century.