More human twins are being born than ever before, mainly due to medically assisted reproduction and women giving birth at an older age, according to the first global overview published on Friday.
Since the 1980s the twinning rate has increased by one-third from 9 to 12 per 1,000 deliveries, meaning about 1.6 million twins are born each year worldwide and one in every 42 children born is a twin.
“A major cause of this increase is the growth in medically assisted reproduction, which includes not only IVF techniques, but also simpler methods, such as ovarian stimulation and artificial insemination,” the study, published in the journal Human Reproduction, concludes.
Another factor is delay in childbearing observed in many countries over recent decades, since the twinning rate increases with the mother’s age.
However, the researchers think the peak in twinning rates may have been reached, particularly in high-income countries in Europe and North America, because of increasing emphasis on the importance of trying to achieve "singleton pregnancies".
Prof Christiaan Monden of the University of Oxford said: "The relative and absolute numbers of twins in the world are higher than they have ever been since the mid-20th 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."
With Prof Gilles Pison of the French Museum of Natural History and Prof Jeroen Smits of Radboud University in the Netherlands, they collected information on twinning rates for the period 2010-2015 from 165 countries, covering 99 per cent of the world's population. For 112 countries, they obtained information on twinning rates for 1980-1985.
They found substantial increases in twinning rates in many European countries, North America and Asia. For 74 per cent out of 112 countries for which data was available for both periods, the increase was more than 10 per cent. There was a 32 per cent increase in Asia and a 71 per cent increase in North America.
Prof Monden said: "In both periods Africa had the highest twinning rates and there was no significant increase over time. However, Europe, North America and the Oceanic countries are catching up rapidly. About 80 per cent of all twin deliveries in the world now take place in Asia and Africa."
The twinning rate in Africa is so high because of high numbers of “dizygotic twins” – born from two separate eggs. This is most likely to be due to genetic differences between the African population and others.
"The absolute number of twin deliveries has increased everywhere except in South America. In North America and Africa, the numbers have increased by more than 80 per cent, and in Africa this increase is almost entirely caused by population growth," Prof Monden said.
Because infant mortality among twins has been going down, many more of the twins born in the second period of the study will grow up as twins compared with those born in the early 1980s.
“However, more attention needs to be paid to the fate of twins in low- and middle-income countries,” Prof Smits noted. “In sub-Saharan Africa in particular, many twins will lose their co-twin in their first year of life, some 200,000-300,000 each year ... 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.”