[Trends in the frequency of twin births]

The proportion of twin delivery has increased by approximately 80% since the beginning of the seventies, due to the combined effects of treatments for sterility, accounting for two thirds of the increase, and delayed childbearing, accounting for one third. The twinning rate had previously experienced upward and downward fluctuations, due to the combined effects of various factors, especially the variations in age at childbearing or voluntary birth control. When the mean age at childbearing increases like over the last 30 years, the twinning rate tends to increase, since older women are more likely to have twins than younger women. On the contrary, when this age decreases like over the first 3 quarters of the 20th century, the twinning rate also tends to decrease. In France, the twinning rate reached a peak during World War I, associated in part with a temporary increase in the mean age at childbearing, as well as with a selection effect of the most fecund couples, which are also more likely to have twins. As for voluntary birth control, it tends to decrease the twinning rate as a result of that selection effect associated with fecundity, but in the opposite direction.