By Kate Kelland
LONDON (Reuters) – More twins are being born now than ever, largely due to the increasing use of in vitro fertilization (IVF) and other assisted reproductive techniques, the first global study of human twins found.
With about 1.6 million twins born each year worldwide, the global rate of twins has increased by a third since the 1980s, to 12 per 1,000 births, from 9 per 1,000 about 30 years ago, concluded the study.
This may well be “peak twins,” said the scientists behind the research – particularly in high-income regions, such as Europe and North America, where there is now an emphasis on refining fertility treatments to minimize multiple births.
“We think we are really at the peak,” said Christiaan Monden, a professor at the University of Oxford in Britain, who co-led the review. “It is likely to be a historic record. The relative and absolute numbers of twins in the world are higher than ever since the mid-twentieth century.”
The Monden research team, whose findings were published on Friday in the journal Human Reproduction, analyzed data on twinning rates for 165 countries between 2010 and 2015 and for 112 countries from 1980 to 1985.
They found a 71% increase in twinning rates in North America, as well as significant increases in many European countries and Asia. For Asia in general, there was an increase of 32%, they said, and only seven countries saw a drop of more than 10% in twinning rates during the study period.
The researchers noted that the rates of monozygotic or identical twins – born from the same egg – have hardly changed, stable at about 4 per 1,000 births worldwide.
This means that the vast majority of the increase in twin rates was due to the high number of dizygotic or non-identical twins – twins born from separate eggs.
This was especially true in Africa, said Monden, and is more likely to be due to genetic differences between African populations and other populations.
“Most of the twins you will meet in Japan are identical twins,” he said, “while most of the twins you will meet in Africa are not identical – and we think this is genetic.”
Although factors such as women’s choice to start a family later, greater use of contraceptives and lower fertility rates may be contributing to the increase in twinning rates, Monden said that assisted reproductive techniques – which started in the 1970s – are the main motivators.
These fertility treatments were originally available in wealthier regions, but they spread to emerging economies in Asia and Latin America in the 1980s and 1990s, reaching the relatively wealthier parts of South Asia and Africa after 2000.
SOURCE: https://bit.ly/2NcUZa7 Human Reproduction, online March 12, 2021.