(WASHINGTON) — The twin birth rate among women age 35 to 39 has jumped by nearly 100 percent since 1980, and among women over age 40, the rate has increased more than 200 percent, a new government study finds.
Epidemiologist Joyce Martin is the lead author on the study, by the National Center for Health Statistics, part of the Centers for Disease Control and Prevention.
“Mothers of all ages and all races and from all parts of the country are having more twins,” said Martin.
The study finds that from 1915 through the 1970’s, the twin birth rate was stable, at about 2 percent of all births. By the early 1980’s though, multiple births began to increase.
“The number of twins more than doubled between 1980 and 2009 and the twin birth rate rose by three-quarters or 76 percent,” Martin said.
If the birth rate had not changed, 865,000 fewer twins would have been born in the U.S. in the last three decades.
Much of the increase can be linked to the success of infertility treatments, but not all. Authors say that women overall are waiting until they are older to have babies, and studies have found that hormonal changes in women over age 35 increase the chance of multiple births.
A 2006 study from the Netherlands found that older women had a higher level of a hormone which stimulates egg production in the ovaries. So older women were more likely to release more than one egg during their monthly cycle.
Still, the study found that, not surprisingly, two-thirds of the increase in twin births in the past few decades is linked to infertility treatment.
“The good news,” said infertility expert Dr. Robert Stillman, is that there are a lot more kids out there from successful fertility therapies, and a lot of them are doing great. But he added, “The bad news is that twins are still complicated.”
Twins are more likely to be born early and at a lower birth weight. Stillman, who is the medical director of Maryland’s Shady Grove Fertility Reproductive Science Center, said 14 percent of twins are born very prematurely, compared with fewer than 2 percent of single births.
Stillman says as infertility treatment improves, the “goal is to eliminate twins.” He added, “The triplet rate and the quadruplet rate has plummeted, and the twin rate is the next thing we have to attack.”
There’s evidence from the new government study that that’s starting to happen. Researchers found that the rate of twin births is still increasing every year, but the annual increase has slowed since 2005.
Stillman says he’s seen evidence of that at Shady Grove, the nation’s busiest infertility center. In 2009, 21 percent of patients trying to get pregnant had a single embryo implanted. In 2010 that jumped to 37 percent, and he expects the 2011 percentage to be even higher.
So while the twin birth rate remains higher than ever, it’s likely to level out in the years ahead.
Copyright 2012 ABC News Radio