By Kate Kelland
LONDON (Reuters) – More twins are being born now than ever before, mainly due to the increasing use of in vitro fertilization (IVF) and other assisted reproduction techniques, the first global study of human twins has found.
With approximately 1.6 million twins born worldwide annually, the global twin rate has risen by a third since the 1980s to 12 per 1,000 deliveries from 9 per 1,000 about 30 years ago, the study found.
It could very well be a peak twin, scientists behind the research said – especially in high-income regions such as Europe and North America, where emphasis is now being placed on refining fertility treatments to reduce multiple births.
“We think we’re actually at the top,” said Christiaan Monden, a professor at Oxford University in Britain. “It will probably be a highlight of all time. The relative and absolute number of twins in the world is higher than it has been since the middle of the twentieth century.”
Monden’s research team, the findings of which were published in the journal Human Reproduction on Friday, analyzed the data on twin rates for 165 countries between 2010 and 2015 and for 112 countries for the period 1980 to 1985.
They found a 71% increase in twin rates in North America, as well as significant increases in many European countries and in Asia. For Asia as a whole, there was a 32% increase, they said, and only seven countries saw a drop of more than 10% in twin rates during the study period.
The researchers noted that the number of monozygotic or identical twins – born from the same egg – had barely changed and was stable at about 4 per 1,000 deliveries worldwide.
This meant that the vast majority of the increase in twin rates was due to the large number of dizzying or non-identical twins – twins born from separate eggs.
This was especially the case in Africa, Monden said, and is probably the result of 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 they are genetic.”
Although factors such as women later choosing to start families, greater use of contraception and lower fertility rates may play a role in the increase in twins, Monden said medically assisted reproductive techniques – which began in the 1970s – are the main drivers.
Such fertility treatments were originally available in richer regions, but spread to emerging economies in Asia and Latin America in the 1980s and 1990s, reaching the relatively richer parts of South Asia after 2000.
SOURCE: https://bit.ly/2NcUZa7 Human reproduction, online March 12, 2021.