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The NCHS released the information spanning 1980 to 2009. Within that time, the twin birth rate rose 76 percent, from 18.9 to 33.3 per 1,000 births.

Joyce Martin, a CDC epidemiologist and co-author of the study told Reuters that the increases are widespread, affecting all age groups and all parts of the country.

For unknown reasons, mothers in their 30s are more likely to have twins than younger or older women, the Associated Press reported. As much as a third of the increase can be attributed to that, Martin told the AP. The rest of the increase is due to fertility treatments.

In 2009, one in every 30 babies born in the United States was a twin. In 1980, one in every 53 was a twin.

The rate was the highest for non-hispanic black women, followed closely by non-hispanic white women. Hispanic mothers continue to be less likely to bear twins, according to the report.

The twin birth rate rose for women of all ages, with the largest increase for women aged 30 and older. According to the report, the rate rose 76 percent for women aged 30-34, nearly 100 percent for women aged 35-39 and more than 200 percent for women aged 40 and older.

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

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