Over the last three decades there has been a rise in the number of twins born in the U.S. Mostly this is the result of test-tube babies and women waiting to have children until their 30s, when the chances of twins increase. In 2009, 1 in every 30 babies born in the U.S. was a twin, an astounding increase over the 1 in 53 rate in 1980, according to a government report issued Wednesday.
Joyce Martin, an epidemiologist who co-authored the Centers for Disease Control and Prevention report said, “The increases are quite widespread, affecting all age groups and all parts of the country…When people say it seems like you see more twins nowadays, they're right.” For some unknown reason, mothers in their 30s are more likely to have twins than younger or older women. As much as a third of the increase can be attributed to that, Martin said. The rest of the rise is due to fertility drugs and treatments. Martin explained, “You have a double whammy going on. There are more older moms and more widespread use of fertility-enhancing therapies.”
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. Twin births are riskier than single births, she said. “Infants born in twin deliveries are at greater risk of poor outcome…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,” Martin said.
Starting in the early 1980s, couples who had trouble conceiving began to benefit from medical advances like fertility drugs and in vitro fertilization (IVF) and other procedures. These treatments became fairly widespread in the 1990s but are expensive, and availability and insurance coverage varies. The twin birth rate rose by more than 2 percent a year, on average, from 1980 through 2004. It leveled off to less than 1 percent annually although the rise from 2008 to 2009 was nearly 2 percent.
In 2009, twin rates increased in all 50 states, though the jumps were highest in lower New England, New Jersey and Hawaii. In Connecticut, twins now account for nearly 5 percent of births. Nationally, 3.3 percent of all births were twins in 2009, up from 2 percent in 1980.
Over the last three decades, rates rose for white, black and Hispanic women, but the increases were not uniform. Rates doubled for whites, rose by half for blacks and by about a third for Hispanics. Historically, black moms have twins most often, but white moms have almost caught up.
“That's changed with infertility treatments,” said Barbara Luke, a Michigan State University expert on twin births. It is seen that the greatest increase in twin rates was for women 40 and older. About 7 percent of all births for women 40 and older were twins, compared to 5 percent of women in their late 30s and 2 percent of women age 24 or younger. These older women are more likely to use fertility treatments and to have two embryos implanted during in vitro fertilization, whereas younger women are more likely to get just one. Some research has suggested women in that age group are more likely to produce multiple eggs in a cycle, increasing their chances of twins.
In October, the practice committees of the ASRM and the Society for Assisted Reproductive Technology (SART) issued a report calling for greater use of “single embryo transfer” in in-vitro fertilization. Studies have shown that pregnancy rates with this approach are high in women who are good candidates. They include those under age 35, having more than one high-quality embryo available, who’ve previously gotten pregnant with in-vitro fertilization, who are in the first or second IVF treatment cycle, and who’ve received donor eggs.
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