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.
They attributed the rest of it to the increase in infertility treatments, specifically in-vitro fertilization and “ovulation stimulation medications.”