Twins may be more common today than at any time in our previous history, says the first comprehensive survey of twin births worldwide, published in Human Reproduction. However, this increase has only been observed with fraternal twins; the rate of identical twin births remains the same.
“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. 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,” Professor Christiaan Monden, of the University of Oxford (U.K.), the first author of the study, said in a statement.
Researchers looked at birth records between 2010-2015 and 1980-1985 for more than 100 countries, and found a significant rise in twin birthrates in the past 40 years, with one in every 42 people being a twin. This is equal to around 1.6 million twin children a year, with the global twin birthrate rising by one-third on average since the 1980s.
The data showed a substantial increase in the number of twins born in North America, Asia, and several European countries. Asia, in particular, observed a 32% increase. However, Africa consistently had the highest rate of fraternal twin births in both time periods, due to genetic differences between the African population and the rest of the world.
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Researchers attribute a major cause of this upswing in twin births to an increase in medically assisted reproduction techniques like IVF (in vitro fertilization), ovarian stimulation, and artificial insemination. Another significant reason is the delay in the age women choose to have children over the last few decades, as there is a correlation between a woman’s age during birth and the potential of conceiving twins. Previous research and trends confirm that millennial women are choosing to give birth later, due to other career and life goals.
“More attention needs to be paid to the fate of twins in low- and middle-income countries. In sub-Saharan Africa in particular, many twins will lose their co-twin in their first year of life, some two to three hundred thousand each year, according to our earlier research. While twinning rates in many rich Western countries are now getting close to those in sub-Saharan Africa, there is a huge difference in the survival chances,” study author Professor Jeroen Smits, of Radboud University in The Netherlands, said in a statement.
Another study author, Professor Gilles Pison of the French Natural History Museum in Paris, adds that while many developed countries are peaking with respect to twin births, developing countries will see a continuing rise for the same. He says, “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.
“Due to their size, India and China will also play a major role. In both cases, medically assisted reproduction might increase twinning rates, while further declines in fertility are more likely in India than in China,” Pison says. “Again, the net result is difficult to predict. Advances in IVF make it more likely than not that in the future we will see fewer twin births as a result of IVF.”
