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 With about 1.6 million twins born each year worldwide, the global
twinning rate has risen by a third since the 1980s, to 12 per 1,000
deliveries from 9 per 1,000 around 30 years ago, the study found.
This might well be "peak twins", scientists behind the research said
- particularly in high-income regions such as Europe and North
America where there is now an emphasis on refining fertility
treatments to minimise multiple births.
"We think we're actually at the peak," said Christiaan Monden, a
professor at Britain's Oxford University who co-led the review.
"This is likely to be an all-time high. The relative and absolute
numbers of twins in the world are higher than they have ever been
since the mid-twentieth century."
Monden's research team, whose findings were published on Friday in
the journal Human Reproduction, analysed data on twinning rates for
165 countries between 2010 and 2015 and for 112 countries for the
period 1980 to 1985.
They found a 71% rise in twinning rates in North America, as well as
significant rises in many European countries and in Asia. For Asia
overall, there was a 32% increase, they said, and only seven
countries saw falls of more than 10% in twinning rates over the
study period.
The researchers noted that rates of monozygotic or identical twins -
born from the same egg - were barely changed, stable at about 4 per
1,000 deliveries worldwide.
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 This meant the vast majority of
the increase in twinning rates was due to high
numbers of dizygotic or non-identical twins -
twins born from separate eggs.
This was especially true in Africa, Monden said, and is most likely
to be due to genetic differences between African populations and
other populations.
"Most twins you'll meet in Japan are identical twins," he said,
"while most twins you'll meet in Africa are non-identical - and we
think that's genetic."
While factors such as women choosing to start families later,
greater use of contraception, and lower fertility rates might be
playing some part in the increase in twinning rates, Monden said
medically assisted reproduction techniques - which began in the
1970s - are the main drivers.
Such fertility treatments were originally available in wealthier
regions, but spread to emerging economies in Asia and Latin America
in the 1980s and 1990s, reaching the relatively richer parts of
South Asia and Africa after 2000.
SOURCE: https://bit.ly/2NcUZa7 Human Reproduction, online March 12.
2021.
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