A population-based study of multiple pregnancy rates in Denmark from 1980 to 1994 found a significant increase in multiple pregnancies, primarily among primiparous women aged 30 years or older. The national incidence of multiple pregnancies increased 1.7-fold over the study period, with the most substantial increase occurring between 1989 and 1994. For primiparous women aged 30 or older, the adjusted twinning rate increased 2.7-fold and the triplet rate 9.1-fold. The adjusted yearly increase in multiple pregnancies for these women was 19%, and for dizygotic twin pregnancies, 25%. The proportion of multiple births among infant deaths in this group increased from 11.5% to 26.9%. However, total infant mortality did not increase due to a significant decrease in infant mortality among singletons. The study attributes these changes to the introduction of new fertility treatments, which have also affected the declining trend in infant mortality. The resources associated with these treatments exceed those invested in specific fertility-enhancing treatments. The study highlights the impact of fertility treatments on multiple pregnancy rates and the associated health implications.A population-based study of multiple pregnancy rates in Denmark from 1980 to 1994 found a significant increase in multiple pregnancies, primarily among primiparous women aged 30 years or older. The national incidence of multiple pregnancies increased 1.7-fold over the study period, with the most substantial increase occurring between 1989 and 1994. For primiparous women aged 30 or older, the adjusted twinning rate increased 2.7-fold and the triplet rate 9.1-fold. The adjusted yearly increase in multiple pregnancies for these women was 19%, and for dizygotic twin pregnancies, 25%. The proportion of multiple births among infant deaths in this group increased from 11.5% to 26.9%. However, total infant mortality did not increase due to a significant decrease in infant mortality among singletons. The study attributes these changes to the introduction of new fertility treatments, which have also affected the declining trend in infant mortality. The resources associated with these treatments exceed those invested in specific fertility-enhancing treatments. The study highlights the impact of fertility treatments on multiple pregnancy rates and the associated health implications.