Peaks in twin births? An international comparison of trends in twinning rates in 30 developed countries

The twinning rate has increased dramatically over the last four decades in developed countries because of two main factors: (a) delayed childbearing, as older women tend to have twins more frequently than younger women, and (b) the expansion of medically assisted reproduction (MAR), which carries an increased risk of multiple births. In some developed countries, however, the twinning rate reached a peak in the early 2000s and has since decreased. This reversal has been observed in more and more developed countries since. It is linked to changes in MAR policies and practices following concerns raised by the increase of the frequency of twin deliveries, twin babies being more fragile than singletons. Using the Human Multiple Births Database (HMBD) and databases on MAR policies and practices, this paper describes twinning-rate trends in 30 developed countries. It examines several questions for particular peaks: at which level did the twinning rate peak in each country? When did it peak? Which factors might explain the recent drop and the lower level attained? For countries that have not yet peaked, when might this occur and at what level