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.
"When people say it seems like you see more twins nowadays, they're right," said Joyce Martin, an epidemiologist who co-authored the Centers for Disease Control and Prevention report.
Some increase was expected as more women are delaying starting a family until they are over 30. 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.
"You have a double whammy going on. There are more older moms and more widespread use of fertility-enhancing therapies," 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 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 theárise 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.
That's high. 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.
The greatest increase in twin rates was for women 40 and older. They 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.
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.
While a lot of attention is focused on the impact of fertility treatments, that's not the only factor. Before fertility treatments existed, about 2.5 percent of the babies born to women in their late 30s were twins, compared to under 2 percent for younger and older women. Some research has suggested women in that age group are more likely to produce multiple eggs in a cycle, increasing their chances of twins.
Clearly, there are more older moms. In recent years, more than a third of all births have been to women 30 and older, up from just one-fifth in 1980.
Take Victoria Jacobson. At 31, she gave birth to twin girls last April -- her first children. With no history of twins in her family, and no fertility drugs, it was a big surprise when she found out at six months pregnant.
"I cried, because I'm more of a realist. My husband laughed," said Jacobson, of Glen Ellyn, Ill. "They were not tears of joy. Maybe shock, more than anything."
Still, Jacobson doesn't find the new report on twin births surprising. A good friend had twins four months after Jacobson's girls were born. "I'm stopped at every corner and in every store" by mothers who also have twins, or who know of other twins, she says.
"I never knew how many twins were around until I got twins."
Are more twins good news? Some experts say the trend is worrisome, noting that multiple births are more dangerous for the mother and their babies. The infants tend to be born earlier, smaller and weaker, and require much more care.
Jacobsons' twins arrived a month early and were tiny, about 3 pounds each. They had to be cared for in a hospital intensive care unit, and one needed a ventilator at first to help her breathe.
At 8 months, they are still small, but healthy and doing fine. Still, they don't sleep through the night, and breastfeeding two and getting them on the same schedule hasn't been easy, she said.
Despite the challenges, she has this advice to other moms of twins: Don't "think about it as double trouble. It's still a double blessing."