In 2014, 33.9 sets of twins were born per 1,000 births in the United States, according to a new report on birth data from the Centers for Disease Control and Prevention.
That’s a very slight increase from 2013, which saw a rate of 33.7 sets of twins per 1,000 births. But, more importantly, it’s the culmination of three decades’ worth of rising twin birth rates—in 1980, only 18.9 sets of twins were born per 1,000 births, according to the CDC.
What’s causing the dramatic rise—and why doesn't the same hold true for births of three or more, the rates for which are down 5 percent between 2013 and 2014?
One reason for the increased twinning rates seems to be the increase in assistive reproductive technologies. These include fertility-boosting medications and artificial insemination, but in-vitro fertilization (IVF) is the most popular.
As the average age of a first-time mother has risen in recent years—in 2014 it was 26.3, compared to 22.7 in 1980)—older moms with reduced fertility have a greater need for fertility-boosting technologies in order to conceive.
Because of how IVF is done, multiple births, like twins or triplets, become more likely. If a couple is having trouble conceiving naturally, scientists can combine the parents’ extracted egg and sperm in the lab, then implant a fertilized embryo in the mother’s uterus.
(More often than not, these embryos won’t take, so hopeful parents have to endure multiple rounds of expensive and emotionally exhausting procedures—the authors of a recent study in the Journal of the American Medical Association found that women were increasingly likely to conceive with each successive round of IVF, up until the ninth round.)
In each cycle, doctors commonly implant more than one embryo in the hope that at least one will be viable. That has caused a huge spike in multiple births—in 2011, 36 percent of all twins in the U.S. were born as a result of fertility treatments including IVF, as the Washington Post notes.
And though doctors are now encouraged to implant no more than two embryos at a time to avoid problematic multiple births (remember Octomom?), and the rate of triplet births has dropped as a result, twins are still more likely to be born as a result of fertility treatments than from natural conception. In 2013, 1.5 percent of all children in the U.S. were born as a result of IVF.
There are, of course, other factors that have led to more twins in the U.S. One may be the sheer fact that mothers are older overall; hormonal changes may make older women more likely to release more than one egg at a time. The social and biological factors that have caused the rising twin rates don’t seem likely to change anytime soon, so, chances are, the trend will continue.
This article originally appeared on Popular Science.
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