Environmental Economics

In 1980, one in every 53 babies born in the United States was a twin. By 2009, that number had risen to one in 30, according to a new brief released by the National Center for Health Statistics. Over the three decades, the twin birth rate (number of twins per 1,000 births) rose 76%.

"It's quite remarkable that the rate has increased as much as it has over this time period," says co-author Joyce Martin, an epidemiologist with the CDC who has studied trends in twins and triplets for decades.

Twin birth rates rose by at least 50% in 43 states and the District of Columbia, and by more than 100% in five: Connecticut, Hawaii, Massachusetts, New Jersey and Rhode Island. The largest increase occurred among non-Hispanic white mothers.

About one-third of the twin birth rate can be explained by an increase in older women having children, according to the brief. Women in their 30s and 40s are naturally more likely to have twins than younger mothers, Martin says. She and her colleagues attribute the rest of the overall increase – nearly two thirds– to the rise of infertility treatments.

Basically, Martin says, "the increase has been pretty widespread. Twins have increased for most age groups and all across the country, across race groups."

Twin births still only make up about 3% of all births, but the increase has some experts concerned.

The study of multiple births is important, the brief notes, because of their elevated health risks and health care costs. Twins are more likely to be born earlier in pregnancy and be smaller than single babies – 1 in 10 twins born between 1980 and 2009 had very low birth weights. More than half are born prematurely, Martin says, and moms of twins and their infants are more likely to be hospitalized longer.

via thechart.blogs.cnn.com