OBJECTIVES: Most currently used age-related risks of T21, T18 and T13 are based on estimates of the live-birth prevalence, and describe an exponential increase of risk by increased maternal age. We investigated the first trimester prevalence of T21, T18 and T13 in a large population of Danish women.

METHODS: From the Danish Cytogenetic Central Registry we got the information of all pre- and postnatally diagnosed fetuses with T21, T18 or T13 between 2005 and 2014 in Denmark. Information on the total number of births and maternal age at birth were gathered from StatBank Denmark.

RESULTS: The total number of included women was 605 853. The total number of T21 cases was 1564, T18 cases was 401 and T13 cases was 157. The overall first trimester prevalence per 10 000 pregnancies was 25.8 for T21, 6.6 for T18 and 2.6 for T13. Boltzmann sigmoidal model (Y = Bottom + (top-bottom / (1 - exp (V50 - X) / slope)) was found to best describe the age-related risk of T21, T18 and T13.

CONCLUSION: We found that the age-related risks are better described by sigmoidal functions, contrary to the widely assumed exponential functions. Our results indicate a lower age-related a priori risk of T21, T18 and T13 compared to widely used risk models. © 2016 John Wiley & Sons, Ltd.