Twins may be more common today than at any time in history, according to the first comprehensive survey of twin births worldwide.
Researchers analyzed records from more than 100 countries and found a substantial increase in twin birth rates since the 1980s, with one in 42 people now being born with twins, the equivalent of 1.6 million children a year. According to the study, the global twin birth rate has increased by an average of one-third over the past 40 years.
Instead of seeing the trend continue, the world may have reached the “twin peak”, say the authors, as the latest data suggests that some countries have started to see a plateau in twinning rates or even a drop from historical maximums.
“The trends are really impressive,” said Christiaan Monden, professor of sociology and demography at the University of Oxford. “In the past 40 to 50 years, we have seen a sharp increase in twinning rates in rich and developed countries, and this has led to more twins in relative and absolute terms than ever before.”
Twins are born naturally when a fertilized egg spontaneously splits in half, leading to identical twins, or when the mother releases two eggs at once that are fertilized, producing non-identical twins. Most are now born as a result of fertility treatments, such as ovarian stimulation or the transfer of more than one IVF embryo to the uterus at the same time, a practice now discouraged in many countries for health reasons.
While the birth rate of identical twins has hardly changed over time, Monden and his colleagues have found that naturally conceived twins, non-identical twins born as a result of medically assisted reproduction – a generic term for a variety of fertility treatments – have increased globally.
The main factors are the increase in access to hormonal treatment, in vitro fertilization and other fertility services, but also the postponement of paternity – the chances of having non-identical natural twins increase with age and reach 35 to 39 years.
The researchers analyzed 2010-15 twinning rates in 165 countries, covering 99% of the world’s population. For 112 of them, they examined additional birth records for 1980-85, according to a Human Reproduction report.
Globally, twinning rates have increased from nine to 12 per 1,000 births since the 1980s, but the picture varies substantially from country to country. The biggest increases are seen in North America (71%), Europe (60%) and Asia (32%). In the UK, twinning rates have increased by about 62%, but are believed to have fallen since the Human Fertilization and Embryology Authority (HFEA) launched a campaign to reduce multiple births in 2007. In the Oxford study, it was found a decrease of more than 10% in just seven countries on the surveyed dates.
Africa has a high rate of twins born naturally from two separate eggs and, according to the study, 80% of twins are now born in Africa or Asia.
“The absolute number of double deliveries has increased everywhere except in South America,” said Monden. “In North America and Africa, the numbers have increased by more than 80%, and in Africa this increase is caused almost entirely by population growth.”
Raj Mathur, president of the British Fertility Society and consultant gynecologist at St Mary’s Hospital in Manchester, said: “It is not surprising that twinning rates have increased because the availability of assisted reproduction has increased and also because women are a little older when they have their first children, and both will increase the rate of twins.
“But at the same time, it is very likely that if you look at Western Europe, particularly the United Kingdom and the Scandinavian countries, you may find that rates are lower than they were in 2010 and 2015. HFEA and the United Kingdom achieved a year- annual reduction in multiple birth rates, which were previously 20% and are now around 10%.
“I think we have reached a peak in terms of twinning rates for medical interventions, certainly in the developed world, but the spread of IVF in Africa and South America is still quite limited on a per capita basis, and there are a large number of people sub-fertile in Africa, particularly who do not have access to in vitro fertilization. The challenge will be how to spread IVF to them without also giving them higher twin rates.
“Most twin babies are absolutely fine, but there is no doubt that a twin pregnancy carries greater risks for both the mother and the baby, so when we can avoid it, we should avoid it. The principle we follow is perfectly summarized by the phrase ‘one at a time’, ”said Mathur.