The maternal age-related first trimester risks for trisomy 21, 18 and 13 based on Danish first trimester data from 2005 to 2014

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. What's Already Known About This Topic? The risk of fetal T21, T18 and T13 increases with maternal age. Various software programs for first trimester risk calculations use age‐specific risk models based on estimations and assuming an exponential increase of risk by maternal age. Morris et al. previously presented data indicating that the live‐birth risk of T21, T18 and T13 does not continue to increase exponentially as assumed. Further results on age‐related risks were demanded. What Does This Study Add? Contrary to previously presented age‐specific risk models, this study is based on high quality first trimester data, and not estimates of the live‐birth prevalence. With this study we present maternal age‐related first trimester risks for T21, T18 and T13 and confirm that the increases follow a sigmoidal curve, opposite the widely assumed exponential curve. Our results indicate a lower a priori risk of T21, T18 and T13 than widely assumed.