Twins may be more common today than ever in history, according to the first comprehensive survey of twin births around the world.
Researchers have analyzed records from more than 100 countries and found that birth rates have risen significantly since the 1980s, with one in 42 people now having twins, equivalent to 1.6 million children a year. According to the study, the global twin birth rate has risen by an average of one-third over the past 40 years.
Instead of seeing the trend continue, the world may have reached a peak twin, the authors say, as the most recent data suggests that some countries have begun to see a platform or even of historical highs daal.
“The trends are really striking,” said Christiaan Monden, a professor of sociology and demography at Oxford University. “Over the past 40 to 50 years, we have seen a sharp increase in twin rates in rich, developed countries, and this has led to more twins in a relative and an absolute sense than we have ever seen before.”
Twins are naturally born when a fertilized egg spontaneously splits in half, resulting in an identical twin, or when the mother releases two eggs simultaneously that are both fertilized, yielding non-identical twins. Most are now born as a result of fertility treatments, such as ovarian stimulation or having more than one in vitro fertilization (IVF) embryo transferred simultaneously into the uterus, a practice that is now discouraged in many countries for health reasons.
While the birth rate for identical twins has barely changed over time, Monden and his colleagues found that a naturally pregnant, non-identical twin and twins born as a result of medically assisted reproduction – an umbrella term for a range fertility treatments – have risen worldwide.
The main drivers are increased access to hormone treatment, IVF and other fertility services, but also the postponement of parenthood – the chances of having a natural, non-identical twin increase with age and reach a peak of 35 to 39 years old.
The researchers analyzed a twin figure between 165 and 15 in 165 countries that covers 99% of the world population. For 112 of them, they examined further birth records for 1980-85, according to a report in Human Reproduction.
Globally, the number of twins has risen from nine to 12 per 1,000 births since the 1980s, but the picture differs significantly from country to country. The largest increases are seen in North America (71%), Europe (60%) and Asia (32%). In the UK, twin rates have risen by 62%, but this is likely to fall since the Human Fertilization and Embryology Authority (HFEA) launched a campaign to reduce multiple births in 2007. In the Oxford study, a decrease of more than 10% was found in only seven countries over the dates examined.
Africa has a high percentage of twins that are naturally born from two separate eggs, and according to the study, 80% of the twins are now born in Africa or Asia.
“The absolute number of twin deliveries has increased everywhere except South America,” Monden said. “In North America and Africa, the numbers have increased by more than 80%, and in Africa this increase is almost entirely caused by population growth.”
Raj Mathur, chairman of the British Fertility Society and a consultant gynecologist at St Mary’s Hospital in Manchester, said: ‘It does not surprise us that the twin rate has increased because the availability of assisted reproduction has increased and also because women are slightly older when they have their first children, and both increase the twin rate.
“But at the same time, it is very likely that if you look at Western Europe, especially the United Kingdom and Scandinavian countries, you can see that the rates are lower than in 2010 and 2015. The HFEA and the British sector have achieved a year – annual decrease in multiple birth rates, which was once at 20% and is now around 10%.
‘I think we have reached a peak in terms of twin rates due to medical interventions, certainly in the developed world, but the spread of IVF in Africa and South America is still quite limited per capita, and there are many infertile people in Africa, especially those that do not have access to IVF. The challenge is to spread IVF to them without giving higher twin rates.
‘The majority of twin babies are absolutely fine, but there is no doubt that a twin pregnancy carries greater risks for the mother and the baby. If we can avoid it, we must avoid it. “The principle we follow is neatly summed up with the phrase ‘one at a time’,” said Mathur.