In a recent study, researchers gathered demographic data from many countries. The goal? To give an overview of the changes that have taken place in the last three decades in the birth of twins.
Today the world is facing a true "twin boom". According to a study published in the journal Human Reproduction on March 12, 2021, approximately 1.6 million pairs of twins come into the world each year, or about one in forty newborns. As Gilles Pison, of the National Institute for Demographic Studies (INED) and one of the study's lead authors, explains, the global twin birth rate has increased by a third since the 1980s , rising from 9.1 to 12.0 per thousand deliveries in just three decades. This boom leads to a worrying situation . Indeed, these twins are often premature, lighter and face more complications during childbirth as well as higher mortality. In addition, the arrival of twins is often complicated for families, who expect to welcome only one child.
The increase in the frequency of twins worldwide is due to the rise in pregnancies of fraternal twins (from two different eggs). Moreover, this unprecedented increase varies according to the continents. With regard to identical twins (monozygotic), they are born everywhere in the same proportions, at the rate of four per thousand deliveries. This increase in multiple births and later pregnancies began in the 1970s with the appearance of medically assisted procreation (MAP). However, the blood level of follicle-stimulating hormone (FSH) acting on the ovum (maturation) and ovulation increases the possibility of having twins until the maximum age of 37 years . Previously, the failure of ovarian function and the increase in embryonic mortality did not allow this.
For several years, advances in assisted reproduction have allowed this success thanks to the implantation of a single embryo. Surplus embryos are frozen. For the researchers, this precaution has made it possible to reach peaks in terms of the twinning rate , especially in rich countries.
Scientists assessed changes in the twinning rate by comparing two periods:1980-1985 and 2010-2015. Of the 1.6 million pairs of twins coming into the world, 1.3 million pairs come from Africa and Asia (in equal parts). The remaining 300,000 pairs are distributed in other areas of the globe.
Asia has a significant number of twins because this continent is home to 60% of the world's population . In the case of Africa, the birth rate is much higher than on other continents. Three decades ago in Europe and North America, the twinning rate was almost half that of Africa. With rates of 14.4 and 16.9 per thousand deliveries, these two countries have now practically reached the very high rate of Africa (17.1).
