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 increase in the twin birth rate, however, does seem to be leveling off. “The 2012 twin birth rate was 33.1 per 1,000 total births, and was essentially unchanged from 2009-2011,” another CDC report found.