Across the globe, the rate of twin births has jumped tremendously over the past 40 years, new research suggests. The study’s authors say that twin births have likely reached an all-time high, thanks largely to the use of medically assisted reproduction, such as in-vitro fertilization, along with more pregnancies in older mothers.
Other studies have shown a clear trend in higher twin pregnancies over time, including in the U.S. But this new study, published this week in the journal Human Reproduction, is one of the first to try figuring out how this trend has played out throughout the world, according to the study authors. They collected birth data from 165 countries (accounting for 99% of the world’s population) from 2010 to 2015, along with data from 112 of those countries from 1980 to 1985.
The results indicate the global twinning rate has increased by a third since the 1980s, from 9.1 twin births per every 1,000 births to 12 twin births per every 1,000. In North America, the rate has climbed even higher, to the point where 3.4% of all births between 2010 and 2015 were twins. In raw numbers, the figures amount to about 1.6 million twins born worldwide every year nowadays. “The absolute and relative number of twins for the world as a whole is peaking at an unprecedented level,” the researchers wrote in the study.
Just as an aside, the team also managed to land an extremely clever title for their paper: “Twin Peaks: more twinning in humans than ever before.”
It’s no secret that in vitro fertilization (IVF) can raise the chances of twin or multiple births, since IVF patients are often implanted with several fertilized embryos to raise the chances of a successful pregnancy. Yet the authors note that other techniques meant to improve fertility, such as ovarian stimulation or artificial insemination, are also likely playing a part. In higher income countries like the U.S., women in general are having children at older ages, and age is known to be a factor in natural twin pregnancies.
Though the rate of twin births increased most everywhere between the 1980s and now, South America was the one major exception and saw a small decline. Meanwhile, the rates in Africa remained the highest and were stable throughout the years, suggesting a more inherent inclination towards twin births in the area, according to the authors.
“The twinning rate in Africa is so high because of the high number of dizygotic twins born there—twins born from two separate eggs. This is most likely to be due to genetic differences between the African population and other populations,” said lead author author Christiaan Monden, a researcher at the University of Oxford in the UK, in a statement released by the journal’s publisher, the European Society of Human Reproduction and Embryology.
As delightful as twins might be, these births are riskier for both the mother and her children. In Africa, where the newborn mortality rate is already higher than elsewhere, many families sadly lose one of their twin children during infancy. In wealthier countries, the rate of survival is higher, but there are still added health risks during and after these pregnancies.
It’s not clear what will happen to the twinning rate moving forward, though the authors speculate that it will gradually decline. Fertility clinics have started to rely more on single-embryo transfers for IVF and other techniques that should reduce the chances of a twin pregnancy. But if countries in Asia or Africa start to experience a decline in fertility that leads to the greater use of medically assisted reproduction, or if women there begin having children at older ages, that could compensate for any declines in twins seen in North America and Europe. For now, the researchers plan to keep an eye on twin trends going into the early 2020s.