Alexis Madrigal notes that there are more twins being born today than there used to be:
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.
The question, obviously, is what’s causing this increase. As it turns out, we are:
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.
They attributed the rest of it to the increase in infertility treatments, specifically in-vitro fertilization and “ovulation stimulation medications.”
The number does appear to be leveling off in recent years, and that seems to be largely attributable to the fact that IVF techniques are becoming more and more sophisticated, which means that doctors are implanting fewer and fewer embryos during each IVF cycle. The fewer embroys implanted, the lower the odds of multiple births. In time, then one expects that the number of twin births will level off as these techniques become more sophisticated, although they will likely remain at a higher rate than they were before the time that artificial insemination techniques became standard.
