There Really Are So Many More Twins Now

A million more, roughly, when compared to the pre-1980 twin rates. So what changed?
Computed from CDC data (Alexis Madrigal)

From about 1915, when the statistical record begins, until 1980, about one in every 50 babies born was a twin, a rate of 2 percent. 

Then, the rate began to increase: by 1995, it was 2.5 percent. The rate surpassed 3 percent in 2001 and hit 3.3 percent in 2010. Now, one out of every 30 babies born is a twin.

That's a lot of "extra" twins above the 1980 baseline, but how many?

When the CDC calculated the number through 2009, they pegged it at 865,000. Now that several years more data is available, I recalculated the number. I took the number of twins that would have been born if the 1980 twin rate had held, and subtracted it from how many twins were actually born. 

The result: 1,009,337! That's a million extra twins from 1981 through 2012, the most recent year for which data is available.

A few years ago, the Centers for Disease Control researchers looked into the phenomenon. Plural pregnancies have an "unfavorable impact on key indicators of perinatal health such as rates of preterm birth and low birthweight." So, they wanted to know: what was causing this large increase in twin births?

The blue portion of the columns indicate how many twin births would have occurred if the twin rate had stayed at the 1980 level. The red portion shows the number of twins born above the 1980 rate for each year (Computed from CDC data/Alexis Madrigal).

Older women tend to have more twins than younger women—and older women are having more of the nation's babies. The researchers found this demographic phenomenon accounted for one-third of the increase.

They attributed the rest of it to the increase in infertility treatments, specifically in-vitro fertilization and "ovulation stimulation medications." 

The increase in the twin birth rate, however, does seem to be leveling off. "The 2012 twin birth rate was 33.1 per 1,000 total births, and was essentially unchanged from 2009-2011," another CDC report found.

Society for Assisted Reproductive Technology.

That is, in part, because assisted reproductive technology procedures have changed. For example, the number of embryos implanted during IVF has a direct relationship with the number of multiple births. So, implanting two (or even three) embryos increases the chance that at least one of them will grow and develop, but simultaneously ups the risk of a plural pregnancy. 

In 2000, three or more embryos were implanted in two-thirds of IVF cycles. During the 2000s, though, the Society for Assisted Reproductive Technology and American Society for Reproductive Medicine published guidelines that recommended fewer embryo transfers, particularly for younger women. So, from 2003 until 2010, one study found that the average number of embryos transferred had fallen from 2.6 to 2. In 10 percent of cases, the prospective parents chose to go with a single embryo transfer, too, which was unheard in earlier decades.

These changes have slowed the increase in the twin birth rate, but have not reversed it yet. Still, every year, more than 50,000 extra twins are still born, joining the million others driving their parents crazy. 

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Alexis C. Madrigal

Alexis Madrigal is the deputy editor of He's the author of Powering the Dream: The History and Promise of Green Technology. More

The New York Observer has called Madrigal "for all intents and purposes, the perfect modern reporter." He co-founded Longshot magazine, a high-speed media experiment that garnered attention from The New York Times, The Wall Street Journal, and the BBC. While at, he built Wired Science into one of the most popular blogs in the world. The site was nominated for best magazine blog by the MPA and best science website in the 2009 Webby Awards. He also co-founded Haiti ReWired, a groundbreaking community dedicated to the discussion of technology, infrastructure, and the future of Haiti.

He's spoken at Stanford, CalTech, Berkeley, SXSW, E3, and the National Renewable Energy Laboratory, and his writing was anthologized in Best Technology Writing 2010 (Yale University Press).

Madrigal is a visiting scholar at the University of California at Berkeley's Office for the History of Science and Technology. Born in Mexico City, he grew up in the exurbs north of Portland, Oregon, and now lives in Oakland.

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