There are more human twins being born than any other time in recorded history, according to the first comprehensive, global study.
Professor Christiaan Monden of the UK’s University of Oxford, along with colleagues Professor Gilles Pison of the French Museum of Natural History and Professor Jeroen Smits of Radboud University in the Netherlands, found that the global twinning rate has increased by a third since the 1980s—jumping from 9 twins per 1,000 deliveries between 1980 and 1985, to 12 twins per 1,000 deliveries between 2010 and 2015.
About 1.6 million twins are now born each year, and one in every 42 children born around the world is a twin.
“The relative and absolute numbers of twins in the world are higher than they have ever been since the mid-twentieth century and this is likely to be an all-time high,” said Professor Monden. “This is important as twin deliveries are associated with higher death rates among babies and children and more complications for mothers and children during pregnancy, and during and after delivery.”
So why the uptick? The study, published Friday in the Human Reproduction medical journal, suggests two major factors.
First and foremost is the growth in medically assisted reproduction (MAR) techniques, such as IVF (in vitro fertilisation), artificial insemination and ovarian stimulation, which have been linked to an increased likelihood of conceiving twins. During procedures like IVF, doctors often implant aspiring mothers with multiple fertilised embryos to increase the chances of at least one successful pregnancy. In some cases, both embryos became viable foetuses, subsequently increasing the global birth rate of twins.
MAR techniques emerged in wealthier countries during the 70s, spreading to emerging economies in Asia and Latin America in the 80s and 90s before eventually reaching more prosperous parts of South Asia and Africa after 2000.
Another, secondary factor in growing twinning rates is the delay in childbearing observed in many countries over the last several decades. Twinning rate increases with the mother’s age, meaning the longer a person waits to fall pregnant the higher the probability of them conceiving twins.
To examine the rising trend, researchers collected information on twinning rates for the 2010 to 2015 period from 165 countries—covering 99 percent of the world’s population—as well as information for the 1980 to 1985 period from 112 countries.
In both periods, Africa had the highest twinning rate and showed no significant increase over time, a pattern that researchers linked to a high number of dizygotic twins—twins born from two separate eggs—most likely as the result of genetic differences in African populations. They further noted that about 80 percent of all twin deliveries in the world now take place between Asia and Africa. But Europe, North America and the Oceanic countries have been catching up.
“The absolute number of twin deliveries has increased everywhere except in South America,” said Professor Monden. “In North America and Africa, the numbers have increased by more than 80 percent, and in Africa this increase is almost entirely caused by population growth.”
Most of the increase in twinning rates comes from dizygotic twins, while there was little change in the rate of monozygotic twins—from the same egg—over time. But researchers believe this may be as prolific as things get.
“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,” said Professor Pison. “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.”
Professor Pison further noted, however, that the net effect of these different drivers is hard to predict. The researchers plan to update their results with data for the early 2020s to examine whether twinning rates have indeed peaked in high income countries, and to measure the effect of MAR techniques becoming more widespread in low and middle income countries.
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