Twin Peaks: more twinning in humans than ever before

Date of news: 12 March 2021

More human twins are being born than ever before, according to a study by researchers at Radboud University, University of Oxford and the French Institute for Demographic Studies (INED).  One in every 42 children born is now a twin, an increase by a third compared to the 1980s. The researchers’ findings are published today in Human Reproduction.

The study shows that the twinning rate has increased by a third from 9 to 12 per 1000 deliveries, meaning that about 1.6 million twins are born every year worldwide, and one in every 42 children born is a twin. A major cause of this increase is the growth in medically assisted reproduction (MAR), which includes techniques like IVF (in vitro fertilisation) ovarian stimulation and artificial insemination. Another cause of the increase is the delay in childbearing observed in many countries over the last decades, since twinning rates increases with the mother’s age.

The researchers think that we may have reached the peak in twinning rates, particularly in high income countries in Europe and North America, because of increasing emphasis on the importance of trying to achieve singleton pregnancies. Whether this is also the case in lower income countries, such as in Africa, is less certain and has important implications for the health of mothers and babies, and also healthcare resources.

“In both periods Africa had the highest twinning rates and there was no significant increase over time. However, Europe, North America and the Oceanic countries are catching up rapidly. About 80% of all twin deliveries in the world now take place in Asia and Africa,” explains professor Pison of the INED.

Global Data Lab

For their study, the researchers collected information on twinning rates for the 2010-2015 period from 165 countries, covering 99% of the world’s population. For 112 countries, they were also able to obtain information on twinning rates for the period 1980-1985.

Most of the data used for low and middle income countries (LMICs) was derived from the Global Data Lab of Radboud University which contains one of the largest databases for LMICs that currently exists, including information on almost 100,000 twin births in Africa, Asia and Latin America.

“This information is very important, as mortality rates among twins are still very high in these countries” explains Jeroen Smits, director of the Global Data Lab and professor of Economic and Human Development at Radboud University. “In sub-Saharan Africa in particular, many twins will lose their co-twin in their first year of life, some two to three hundred thousand each year according to our earlier research.”

Death rates and complications

Professor Christiaan Monden of the University of Oxford (UK) said: “The relative and absolute numbers of twins in the world are higher than they have ever been since the mid-twentieth century and this is likely to be an all-time high. This is important as twin deliveries are associated with higher death rates among babies and children and more complications for mothers and children during pregnancy, and during and after delivery.”

The researchers say that countries need to keep better data on twin births, particular lower income countries where deaths among twins is highest, as reliable statistics are lacking in many places. They plan to update their results with data for the early 2020s to see whether twinning rates have indeed peaked and started to decrease in high income countries, and what will be the effect of the wider spread of medically-assisted reproduction in low and middle income countries.

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