"The increases are quite widespread, affecting all age groups and all parts of the country," said Joyce Martin, a CDC epidemiologist and coauthor of the new study.

More than 137,000 twins were born in the United States in 2009, accounting for one in every 30 babies. That compares to 68,339 twins born in 1980 when just one in 53 infants born was a twin, the CDC said.

A third of the increase in the twin birth rate can be attributed to women waiting longer to have children, the CDC said. From 2000 to 2009, more than 35 percent of all births were to mothers ages 30 and over, up from 20 percent in 1980.

The number of twins per 1,000 births rose in all 50 states and doubled in Connecticut, Hawaii, Massachusetts, New Jersey and Rhode Island.

Treatment for infertility such as in-vitro fertilization accounts for much of the remainder of the increase in twins, the CDC said.

"We seem to be making improvements, refinements to fertility-enhancing therapies, so that could then result in a lowering of the increase of the pace in twin and other multiple births," Martin told Reuters.

Twin births are riskier than single births, she said.

"Infants born in twin deliveries are at greater risk of poor outcome," she said. "They are born smaller, they are born earlier. They are more likely not to survive the first year. Most twins do fine, but they are at higher risk."

(Editing by Colleen Jenkins and Cynthia Johnston)

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