No, you are not seeing double—more twins are being born than ever before. Between 1980 and 2015, twin birth rates increased by 42 percent, resulting in an average 1.6 million twins born annually in the 2010s, reports Rachael Rettner for Live Science.
The soaring rates may be a result of increased popularity in using fertility treatments, and more people deciding to have children later in life, according to a new paper published in the journal Human Reproduction last week. The study is the first comprehensive global analysis of twin birth rates.
The researchers compared data collected between 1980 to 1985 from 112 countries to numbers from 2010 to 2015 in 165 nations. Globally, twin birth rates increased from 9 to 12 twins for every 1,000 births between the early 1980s and the early 2010s, reports Rory Sullivan for the Independent.
Wealthy continents had the highest increase in twin birth rates, such as North America with a 71 percent increase, Europe with a 58 percent increase, and Oceania with a 46 percent increase, Live Science reports. The surges in twin births occurred only in fraternal twins, or dizygotic twins, where two eggs were separately fertilized. Birth rates of identical twins, or monozygotic twins, where one egg is fertilized but splits into two eggs, remained the same at four identical births per 1,000 births, Live Science reports.
Since the first successful in vitro fertilization (IVF) birth in 1978, where an egg is fertilized with sperm outside of the body in a lab setting and later transferred into the uterus, the procedure accounts for millions of births worldwide. In the U.S. and Europe alone, IVF accounts for one to three percent of all births each year.
Increases in medically-assisted reproduction methods, including IVF, may be influencing the rise of twin births. During these procedures, more than one embryo is implanted to improve the chances that at least one will survive, reports Live Science. Ovarian stimulation—hormones are administered to stimulate the release of multiple eggs—can also increase the chances of having twins, reports Clare Wilson for New Scientist.
Although more twins are born each year from fertilization methods in wealthy countries, the researchers suggested these numbers reached a peak and expect them to drop soon as IVF methods become more advanced and the need more implanting multiple embryos is no longer needed, the Independent reports.
Another reason the authors suspect there is an increase in twin births is that more adults are choosing to have children later in life. Financial considerations, career, and life goals—or even the Covid-19 pandemic—are a few reasons why people may delay having children, Ashley Stahl reported in 2020 for Forbes.
Having children later in life also raises the chances multiple eggs will be released at once during ovulation, making conceiving twins more likely, New Scientist reports. Ovaries release numerous eggs to compensate for declining fertility as the body ages and to counteract the increased risk of early fetal loss, according to a study in Nature published in 2020.
Overall, twin births rose on a global scale, except in South America, the Independent reports. Africa remained the continent with the highest twin birth rate at 17 to every 1,000 births. Still, these numbers remained unchanged throughout the three-decade period the study looked at and may be due to populations in Africa being more genetically prone to having twins, Live Science reports.
In future studies, the researchers hope to collect more data on low- and middle-income countries. When new data from the 2020s becomes available, the team predicts twin births could peak in wealthy countries. As medically-assisted reproduction methods become more accessible in increase in low- and middle-income countries, the researchers expect to see twin birth rates spike in the next ten years.
“Most data suggest we are at a peak in high-income countries, especially Europe and North America. Africa will be one of the main drivers in the coming decades,” says study co-author Gilles Pison, a researcher at the French Museum of Natural History, in a statement. “We might see a combination of lower overall fertility, older ages at birth, and more medically assisted reproduction. The former would lead to lower twinning rates, the latter two to higher twinning rates. The net effect of these different drivers is uncertain.”