In many developed countries, the rate of twin births has doubled or more than doubled over the past few decades, according to a recent study published in Population and Development Review.
In 1975, there were 9.5 twin births per 1,000 deliveries in the United States. In 2011, there were 16.9 twins per 1,000 births. The increase over that time was similar in England and Wales (from 9.9 to 16.1), France (9.3 to 17.4), and Germany (9.2 to 17.2), and a little steeper in Denmark (9.6 to 21.2) and South Korea (5 to 14.6).
This rise is pretty much entirely from fraternal twins—two eggs released during ovulation and fertilized by two different sperm. The rate of identical twins, who develop from one zygote that splits and forms two embryos, tends to stay relatively constant around the world, and doesn’t seem to be affected by any external factors. But the likelihood of having fraternal twins, or dizygotic twins, changes depending on the age of the mother, how many children she’s already had, the country she lives in, and genetic factors.
Twin Rate in Developed Countries, 1900 to 2013
The older the mother, the more likely she is to have fraternal twins. This is one reason the study cites for the increase of twins in wealthy, developed countries, where women are more likely to have their first child later in life. In the United States, the average age of first-time motherhood was 26.3 in 2014, compared to 24.9 in 2000. In other countries, particularly in Europe and Southeast Asia, the average age is even older—Greece holds the top spot at 31.2, according to the Central Intelligence Agency’s World Factbook.