A massive global study tracking birth data from 165 countries has found more twins are being born than ever before. The study reveals twinning rates rose by a third over the past 40 years but the researchers suggest this trend may have reached a natural peak.
Over the 20th century global twinning rates increased due to a number of factors. From women bearing children at older ages, to the rise in medically assisted reproductive methods, rates of twin births have unsurprisingly grown in most regions of the world.
A new study published in the journal Human Reproduction offers an up-to-date global investigation of twin births. The robust research collected data from 165 countries covering twinning rates between 2010 and 2015. This was compared to similar data looking at global twinning rates in the early 1980s.
The overall global twinning rate was found to have risen by about 30 percent since the 1980s, growing from nine in 1,000 births to 12 per 1,000. The study calculates one in every 42 children born is a twin.
“The relative and absolute numbers of twins in the world are higher than they have ever been since the mid-twentieth century and this is likely to be an all-time high,” says Christiaan Monden, first author on the new study.
In almost all countries studied the researchers found twinning rates have increased over the past few decades. Only South America was found to show an absolute decrease in twin births since the 1980s.
“In both periods Africa had the highest twinning rates and there was no significant increase over time,” says Monden. “However, Europe, North America and the Oceanic countries are catching up rapidly. About 80 percent of all twin deliveries in the world now take place in Asia and Africa.”
Monden hypothesizes a number of reasons to explain why Africa’s rates of twin births are so high. Improvements to maternal health care, increased use of contraception methods leading to older maternal age, and a growing access to reproductive technologies all play a role but he also suggests fundamental genetic differences could influence the high twinning rates in Africa.
“The twinning rate in Africa is so high because of the high number of dizygotic twins born there – twins born from two separate eggs,” suggests Monden. “This is most likely to be due to genetic differences between the African population and other populations.”
Another author on the study, Gilles Pison from the French Museum of Natural History, hypothesizes these findings to possibly be a global peak in twinning rates. High-income countries especially are potentially hitting their peak in twinning rates, with advances in IVF technology hypothesized to lead to fewer twin births in the future.
“Most data suggest we are at a peak in high income countries, especially Europe and North America," says Pison. "Africa will be one of the main drivers in the coming decades. We might see a combination of lower overall fertility, older ages at birth and more medically assisted reproduction.”
The study was published in the journal Human Reproduction.
Source: Radboud University