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Around the world, an unprecedented explosion of twin births



There have never been so many births of twins in the world: a peak explained by the extension of ART and the later age of pregnancies, according to a study published on Friday March 12 in the specialist journal Human Reproduction.

More than 1.6 million pairs of twins are born each year in the world, i.e. “Almost one in 40 babies”, argue the researchers behind the study. Since the 1980s, the global rate of twin births has increased by a third, from 9.1 to 12.0 per 1,000 births, in just three decades, says Gilles Pison, one of the authors of the study, professor at the National Museum of Natural History and associate researcher at the National Institute of Demographic Studies (INED).

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The rise in the worldwide frequency of twins is due solely to the unprecedented rise in pregnancies of so-called twins. ” fake twins “ (from two different eggs). Identical twins, known as monozygotes, are born everywhere in the same proportions, with “A constant rate – of four births of identical twins per thousand births – not varying with the age of the woman, nor from one region to another”, notes Gilles Pison.

Strong increase in Europe and North America

Of the 3.2 million twins born each year, 1.3 million are born in Africa (or 650,000 pairs) and as many in Asia, which brings together 60% of humanity. The large number of twins in Africa (which represents 17% of humans) is explained by a much higher birth rate, which is between double and triple that of other continents as well as by a highest starting twinning rate in the world.

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On other continents, the number of twins stands at 600,000. The twinning rate in Europe and North America thirty years ago was almost half that in Africa, but has since increased significantly (reaching 14.4 and 16.9 deliveries of twins per 1,000 respectively) and almost joined that of Africa, which has hardly changed (17.1).

Technical progress of PMA

Medically Assisted Reproduction (ART), which began in wealthy countries in the 1970s, has contributed to this increase in multiple births, as have later pregnancies. Technical progress in assisted reproduction has made it possible for several years to have a pregnancy with just as much success by implanting only one embryo and freezing those in excess. A precaution that makes “We may have reached a peak in terms of twinning rates [dans ces pays] “, note the authors.

The blood level of a hormone that intervenes in the maturation of the egg and ovulation, FSH, increases with age and explains that the probability of a twin pregnancy increases until it reaches a maximum at 37 years. Beyond that, before the onset of ART, the rate of fraternal twins decreases rapidly due to failure of ovarian function and an increase in embryonic mortality.

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