Some increase was expected as more women are waiting until they are over 30 to have babies. 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, experts said.
"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 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.
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 are to women 30 and older, up from just one-fifth in 1980.
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.
But for some older women worried about conceiving, "having twins is a blessing," Luke said.
CDC report: http://www.cdc.gov/nchs/