"The Epidemic of Multiple Pregnancies"

The rise in twin, triplet, and higher order births in the United States has been called an epidemic, according to the National Center for Health Statistics. Until 1980, the incidence of multiple births was stable at about 2 percent of births--or 1 in every 53 babies. Then things began to change. By 2009, 1 in every 30 babies born was a twin. The rise in the twin birth rate resulted in an additional 865,000 more twins being born over the three decades than would have been born if the rate had remained stable.

Behind the increase in twinning is the use of fertility treatments and the older age of women at childbirth. Among women aged 40 or older, the twin birth rate climbed by more than 200 percent between 1980 and 2009, with twins now accounting for fully 7 percent of births in the age group. Most of the rise in twinning--two-thirds--is the result of the use of fertility treatments, according to the government's analysis. The remainder of the increase is due to the older age of women at childbirth.

Source: National Center for Health Statistics, Three Decades of Twin Births in the United States, 1980-2009