Secular trends in twinning rates

Based on national data from civil birth registration systems, this paper provides an overview of trends in the twinning birth rates in fifteen developed countries. Patterns and differentials in such rates across populations are described, and trends over time are given. The evolution of the twinning rate in industrialized countries over the last century follows broadly the same pattern. One exception is the period around World War I, with a peak in the twinning rate being observed during the war, or just after it, although this was not registered in all countries. Since the mid-1970s twinning rates have increased in many developed countries in response to a growing use of fertility-stimulating treatments such as in vitro fertilization.