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Most developed countries have good data on twinning and multiple birth rates. In this study, researchers in France and Italy used data from civil birth registries to review twinning rates during the past century in 19 developed countries (mostly in Europe and not including the U.S.).
Twinning rates remained relatively constant through the first half of the 20th century (about 13 per 1000 deliveries). From the 1950s until the 1970s, as urbanization increased, twinning rates fell to about 9 per 1000 births. Advanced maternal age at childbirth, which is associated with twinning, contributed to a gradual rise in twinning until the mid-1990s, followed by a dramatic and almost exponential increase (as high as 18–20 per 1000 deliveries in some coun…