Twin birth rate rose 76 percent from 1980 through 2009.

Over the past three decades, the number of twins born to American women has risen dramatically, a new government study shows.

The twin birth rate rose 76 percent from 1980 through 2009, according to report from the Centers of Disease Control and Prevention that was released Wednesday. While 189 out of every 10,000 births was a twin delivery in 1980, in 2009, 333 out of 10,000 births involved twins.

Put another way, in 2009 one in every 30 babies born in the United States was a twin, as compared to one in every 53 babies born in 1980.

Researchers say that the uptick in twin births is due to both the increased use of infertility treatments and the tendency for women to delay child-birth till they are older.

The increase in twin births is concerning, said the study’s lead author Joyce A. Martin, an epidemiologist at the CDC’s National Center for Health Statistics.

“It’s really important to note that outcome for twins is much less positive than for singleton pregnancies,” Martin said. “Twins tend to be born earlier and smaller.. Their mothers are more likely to require hospitalization. And the twins themselves are more likely than singletons to require hospitalization.”

But, Martin added, “although they are at greater risk overall, most twin births do very well.”

The rate of twinning isn’t the same all over the country. In New Mexico, for example, 223 out of 10,000 births were twin deliveries. But in Connecticut the number was far higher: 459 out of 10,000 births.

The increases seen by Martin and her colleagues also varied by race and ethnic group. The biggest increase in twins was among white non-Hispanic mothers. For them, the rate of twins doubled over the past three decades.

Interestingly, black women have had a higher rate of twinning all along. Nobody knows why this is, but experts assume it’s mostly explained by genes, just as the tendency to twin seems to run in certain families, said Dr. Joseph Sanfilippo, a professor of obstetrics, gynecology and reproductive sciences and director of reproductive endocrinology and infertility at the University of Pittsburgh Medical Center.

It’s clear that there are big variations in women’s tendency to have twins, Sanfilippo said. That can be due to genetics and environment. Rates vary widely from country to country, Sanfilippo said. Yoruba has the highest rate with 450 to 500 sets of twins per 10,000, he added.

The big increases Martin and her colleagues have seen in the U.S. in white non-Hispanic women have almost caught them up with black women.

While the rise in the rate of twins occurred in every age group, the biggest surges were in women 30 and older. From 1980 to 2009, the rate of twins increased 76 percent among women aged 30 to 34 and nearly 100 percent for women aged 35 to 39. Among women aged 40 and older the rate surged more than 200 percent, the Martin and her colleagues reported.

Nobody knows exactly why older women are more prone to twinning, Sanfilippo said. But the most likely explanation is that older women have older eggs.

While these older eggs still have DNA that is intact enough to produce a healthy baby, the mechanisms that allow the fertilized egg to grow and divide may be somewhat compromised and this is what may lead to more twinning.

“The egg is the orchestra leader,” Sanfilippo explained. “If it’s been sitting around in the ovaries for 35 to 40 years it doesn’t work as well. Certainly not as well as one that is just 25 years old.”

Ultimately older moms are only responsible for one third of the increase in twins, Martin said. The rest of the surge is due to increasing use of infertility therapies like in vitro fertilization.




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