Joyce A. Martin, Brady E. Hamilton and Michelle J.K. Osterman of the National Center for Health Statistics -- part of the Centers for Disease Control and Prevention -- said the twin birth rate rose 76 percent from 1980 through 2009, from 18.9 to 33.3 per 1,000 births, mostly due to mothers having children at an older age and getting fertility treatments.
Twin birth rates rose by nearly 100 percent over three decades among women ages 35-39 and more than 200 percent among women aged 40 and older, the report said.
The older age of women at childbirth in 2009, compared with that of three decades earlier, accounts for only about one-third of the rise in twinning over the 30 years.
The incidence of multiple births in the United States was stable at about 2 percent of all births from 1915 through the 1970s -- but twinning rates, and those for triplet and higher-order multiples, began to rise in the early '80s, leading to what has been called "an epidemic of multiple pregnancies," the researchers said.
"Plural pregnancies tend to exact a greater toll on the health of the mother and outcomes for births in multiple deliveries are often compromised compared with singletons," the report said.