More twins are being born now than ever before – mainly because of the rising use of IVF and more people starting their families later in life. The rate of twin births may now be at its peak, though, due to fertility clinics refining their techniques.
The global rate of twin births has risen by a third since the 1980s, from 9 per 1000 births to 12 per 1000. Christiaan Monden at the University of Oxford and his colleagues generated the figures by gathering existing data from 165 countries from 1980 to 2015. Twins are also more likely to survive now, thanks to medical advances. “The rates are higher than they have been for 50 years,” says Monden. “This is likely to be an all-time high.”
Most of the increase has been in fraternal or non-identical twins, who develop from separate eggs and sperm. The rate of identical twins, caused by an embryo splitting in two in the first few days after fertilisation, has stayed about the same.
Growing use of fertility treatments is probably the biggest factor behind the rise in twin births in high and middle-income countries, says Monden. Women may take hormones to stimulate egg production, which can lead to them releasing two eggs at once.
Also, IVF clinics may transfer two, three or more embryos into a uterus simultaneously, to boost the chances that at least one survives. This can lead to pregnancies of twins, triplets or even higher numbers of babies.
Because babies in such pregnancies have greater health risks, such as being born prematurely and underweight, many guidelines from regulators, such as in the UK, now say fertility clinics should aim to transfer only one embryo in each attempted pregnancy cycle. This could mean the rate of twin births will now start to drop.
Other factors contributing to the rise in twin births over the past three decades include women having children later in life in high and middle-income countries. Older women are more likely to release two eggs at the same time.
There has also been population growth in Africa, which has long had a relatively high rate of fraternal twin births. For instance in the 1980s, the twin birth rate in sub-Saharan Africa was about twice that in the UK, although the UK has since caught up. The higher rate in Africa is thought to be due to unknown genetic factors, says Monden.
Journal reference: Human Reproduction, DOI: 10.1093/humrep/deab029
Sign up to our free Health Check newsletter for a round-up of all the health and fitness news you need to know, every Saturday
More on these topics: