Twins are being born more and more in the world, according to a new global study. Professor Christiaan Monden of the University of Oxford in the United Kingdom, together with his colleagues, Professor Gilles Pison of the French Museum of Natural History and Professor Jeroen Smits of Radboud University in the Netherlands, found that the birth rate of Overall twins have increased by a third since the 1980s, from 9 twins per 1,000 deliveries between 1980 and 1985, to 12 twins per 1,000 deliveries between 2010 and 2015.
Approximately 1.6 million twins are born each year, and one in 42 children born worldwide is a twin.
“The relative and absolute number of twins in the world is higher than it has been since the mid-20th century and is likely to be an all-time high,” said Professor Monden. “This is important as twin deliveries are associated with higher infant and child mortality rates and more complications for mothers and children during pregnancy, and during and after delivery.”
So why the rebound? The study, published in early March in the medical journal Human Reproduction, suggests two main factors.
First, there is the increase in assisted reproductive techniques, such as in vitro fertilization (IVF), artificial insemination and ovarian stimulation, which they have related with a higher chance of conceiving twins. During procedures like IVF, doctors often implant multiple fertilized embryos into mothers to increase the chances of at least one successful pregnancy. In some cases, both embryos develop into viable fetuses, this has caused the overall twin birth rate to increase.
Assisted reproductive techniques emerged in the richest countries during the 1970s and spread to the emerging economies of Asia and Latin America in the 1980s and 1990s before finally reaching more prosperous parts of South Asia and Africa after the 1970s. year 2000.
Another secondary factor in the increase in the twin birth rate is the delay in childbearing observed in many countries over the past decades. The twin rate increases with the mother’s age, which means that the longer a person waits to get pregnant, the greater the chance that they will conceive twins.
To examine this increasing trend, the researchers collected information on twin birth rates for the period 2010 to 2015 from 165 countries, covering 99 percent of the world’s population, as well as information for the period 1980 to 1985 from 112 countries.
In both periods, Africa had the highest twin rate and did not show a significant increase, a pattern that the researchers linked to a high number of dizygotic twins (twins born from two separate eggs) likely as a result of genetic differences in African populations. . Furthermore, they noted that around 80 percent of all twin deliveries in the world now take place between Asia and Africa. But Europe, North America and the oceanic countries have been catching up.
“The number of twin deliveries has increased everywhere except South America,” said Professor Monden. “In North America and Africa, the numbers have increased by more than 80 percent, and in Africa this increase is due almost entirely to population growth.”
“Most of the data suggests that we are at a peak in high-income countries, especially in Europe and North America. Africa will be one of the main drivers for decades to come, ”said Professor Pison. “We could see a combination of lower fertility in general, older ages at birth and more assisted reproduction. The former would lead to lower twin birth rates, the latter two to higher rates. “
Professor Pison further noted that the net effect of these different factors is difficult to predict. The researchers plan to update their results by the early 2020s to examine whether twin birth rates have actually peaked in high-income countries and to measure the effect of increasingly widespread assisted reproductive techniques in those countries. low and middle income countries.
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