World at 'peak twin' with 1.6 million twins now born every year: Oxford study

The increased number of twins, especially in poorer parts of the world, is concerning, because a twin pregnancy carries greater risks for the mother and the baby, say researchers.

More twins are born today than at any time in history. As many as 1.6 million twins are now born every year, reveals the first comprehensive survey of twin births around the world carried out by the Oxford University. The study that analysed records from more than 100 countries found a substantial rise in twin birth rates since the 1980s. It said that the global twin birth rate has risen by one-third, on average, over the past 40 years, with one in 42 children now born a twin, which is equivalent to 1.6 million children a year. The findings were published in the scientific journal Human Reproduction.

The authors of the study noted that the world may have reached "peak twin", as some countries have begun to see twinning rates plateau or even fall from historic highs the Guardian reported.

"Over the past 40 to 50 years we've seen a strong increase in twinning rates in rich, developed countries, and that has led to more twins in both a relative and an absolute sense than we've seen ever before," Christiaan Monden, a professor of sociology and demography at Oxford University, was quoted as saying.

Twin birth rate highest in North America

For the study, the researchers analysed twinning rates from 2010-15 in 165 countries. Global twinning rates have risen from nine to 12 per 1,000 births since the 1980s.The highest twin birth rate is seen in North America (71%), followed by Europe (60%) and Asia (32%). Some countries have started seeing fall from historic highs. For example, the UK twinning rates rose about 62% but fallen again since the Human Fertilisation and Embryology Authority (HFEA) launched a campaign to reduce multiple births in 2007, the study noted. Only seven countries recorded decrease of more than 10% over the dates surveyed.

In terms of twins born naturally from two separate eggs, Africa has the highest rate, which is caused almost entirely by population growth, according to the study.

Reasons for rise in twin birth rates

Naturally, twin birth happens when a fertilised egg splits in half, leading to identical twins, or two eggs are fertilised at once, producing non-identical twins. Today, most twins are born as a result of fertility treatments, such as ovarian stimulation or transferring more than one in vitro fertilisation (IVF) embryo into the womb at the same time.

According to Monden and his team, the birth rate for identical twins has barely changed over time, but naturally conceived, non-identical twins and twins born as a result of medically assisted reproduction had risen globally. The main factors that contributed to the rise in multiple births include:

• Increased access to hormone treatment, IVF and other fertility services. There has been a rise in use of assisted reproductive technology (ART) in developed countries since the 1970s.
• Postponement of parenthood mother giving birth at an older age have higher chances of having natural, non-identical twins. The risk peaks at 35 to 39 years old.

The increased number of twins, especially in poorer parts of the world, is concerning, said the researchers. This is because a twin pregnancy carries greater risks for the mother and the baby. Twin deliveries are associated with higher death rates among babies and children, they noted.