More human twins are being born than ever, mainly due to medically assisted reproduction and women giving birth at a later age, according to the first summary published Friday.
Since the 1980s, the twin rate has increased by a third from 9 to 12 per 1,000 births, meaning that around 1.6 million twins are born worldwide each year and one in 42 children born. is a twin.
“One of the main causes of this increase is the growth of medically assisted reproduction, which includes not only in vitro fertilization techniques, but also simpler methods, such as ovarian stimulation and artificial insemination”, concludes the study, published in Human Reproduction magazine. .
Another factor is the delay in childbearing observed in many countries in recent decades, as the twinning rate increases with the age of the mother.
However, researchers believe that peak twinning rates may have been reached, particularly in high-income countries in Europe and North America, due to the growing emphasis on the importance of trying to achieve “singleton pregnancies.”
Professor Christiaan Monden of the University of Oxford said: “The relative and absolute number of twins in the world is greater than ever since the middle of the 20th century, and it is likely to be a historical record. This is important because twin births are associated with higher infant and child mortality rates and more complications for mothers and children during pregnancy, and during and after delivery. “
Substantial increases
With Professor Gilles Pison of the French Museum of Natural History and Professor Jeroen Smits of Radboud University in the Netherlands, they collected information on twinning rates for the period 2010-2015 from 165 countries, covering 99 percent of The world population. For 112 countries, they obtained information on twinning rates in 1980-1985.
They found substantial increases in twinning rates in many European countries, North America, and Asia. For 74% of the 112 countries for which data were available for both periods, the increase was more than 10%. There was an increase of 32% in Asia and 71% in North America.
Professor Monden said: “In both periods, Africa had the highest twinning rates and there has not been a significant increase over time. However, Europe, North America and the oceanic countries are recovering rapidly. About 80 percent of all twin births in the world now occur in Asia and Africa. “
The twin rate in Africa is very high due to the large number of “dizygotic twins”, born from two separate eggs. This is more likely due to genetic differences between the African population and others.
“The absolute number of double births has increased everywhere except South America. In North America and Africa, the numbers have increased by more than 80 percent, and in Africa this increase is almost entirely due to population growth, ”said Professor Monden.
As infant mortality among twins has decreased, many more twins born in the second period of the study will grow up as twins compared to those born in the early 1980s.
“However, more attention needs to be paid to the fate of twins in low- and middle-income countries,” said Professor Smits. “In sub-Saharan Africa in particular, many twins will lose their co-twins in the first year of life, between 200,000 and 300,000 each year… While twinning rates in many wealthy Western countries now approach those of the South African Sahara, there are a big difference in the chances of survival. “
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