The science behind the recent surge in twins and triplets

The rate of twin births in the United States has increased nearly 80% since 1980, according to a report from the Centers for Disease Control and Prevention.

In 2013, 33.7 of every 1,000 births were twins, compared to just 18.9 out of 1,000 in 1980. Most of that increase occurred between 1980 and 2009, when the rate of twin births rose 76%.

The rate of triplets and higher order multiple births (like quintuplets), while rarer overall, has also increased dramatically since 1980 - it's more than quadrupled. Some researchers have gone so far as to call this spike in multiple births an "epidemic."

The graph below shows the overall rate of multiple births in the United States since 1980:

Andy Kiersz

Research suggests that the high rate of multiple births in the US is due to more women waiting to have children and using fertility treatments like in vitro fertilization (IVF) when they do. By 2011, as many as 36% of twins were conceived with medical assistance, scientists estimate.

When a woman gets IVF to try to conceive, one or more embryos fertilized in a lab are transferred to her uterus, where at least one hopefully will implant and grow into a baby. To increase the chances that a woman will become pregnant, doctors often transfer more than one embryo. At the same time, this increases the chances that a woman will become pregnant with more than one fetus, then give birth to multiples.

The risk is now well-known, though not entirely avoided. As researchers noted in The New England Journal of Medicine, "though the practice of transferring three or more embryos in a single cycle has declined steadily, several factors may constrain efforts at further reduction."

Flickr/Allan Foster

Exactly which part of assisted reproduction is driving the twins surge is not entirely settled, however. According to the American Society for Reproductive Medicine, it's actually the use of fertility drugs that's responsible for most of the increase in multiple births. Normally, only one egg from a woman's ovaries is available for fertilization in a month, but fertility drugs work by stimulating the ovaries so more than one egg becomes available. With more eggs around for fertilization, women are more likely to become pregnant with multiples.

Also, though fertility generally decreases as women age, the rate of multiple births is higher for older women. In the US, birth rates have been increasing for women over 30, according to the same CDC report, and this may also have something to do with the rising multiple birth rate.

The multiple birth rate's precipitous increase since 1980 looks pretty strange at first, but there's clear science behind it.