From about 1915, when the statistical record begins, until 1980, about one in every 50 babies born was a twin, a rate of 2 percent.
Then, the rate began to increase: by 1995, it was 2.5 percent. The rate surpassed 3 percent in 2001 and hit 3.3 percent in 2010. Now, one out of every 30 babies born is a twin.
That's a lot of "extra" twins above the 1980 baseline, but how many?
When the CDC calculated the number through 2009, they pegged it at 865,000. Now that several years more data is available, I recalculated the number. I took the number of twins that would have been born if the 1980 twin rate had held, and subtracted it from how many twins were actually born.
The result: 1,009,337! That's a million extra twins from 1981 through 2012, the most recent year for which data is available.
A few years ago, the Centers for Disease Control researchers looked into the phenomenon. Plural pregnancies have an "unfavorable impact on key indicators of perinatal health such as rates of preterm birth and low birthweight." So, they wanted to know: what was causing this large increase in twin births?
Older women tend to have more twins than younger women—and older women are having more of the nation's babies. The researchers found this demographic phenomenon accounted for one-third of the increase.