The number of human twins being born has increased by one third in the last 40 years – the most ever recorded, according to an analysis published today.
In what is the first comprehensive, global overview published today (12 March, 2021), the study finds a major cause of the increase is the growth in medically assisted reproduction (MAR), which includes IVF techniques as well as ovarian stimulation and artificial insemination.
The study, published in Human Reproduction, also says delay in childbearing is another factor, because the older a mother is, the greater the chance of conceiving twins.
The University of Oxford, UK, led research has found since the 1980s the twinning rate has increased by a third from nine to 12 per 1000 deliveries, equating to about 1.6 million twins being born each year worldwide.
However, the team believes “peak twinning rates” have been reached, particularly in high income countries such as Europe and North America, because of increasing emphasis on the importance of trying to achieve singleton pregnancies. It is less certain if this is the case in lower income countries.
First author Professor Christiaan Monden 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, including Professor Gilles Pison of the French Museum of Natural History in Paris, France, and Professor Jeroen Smits of Radboud University in The Netherlands, collected information on twinning rates from 2010-2015 in 165 countries.
They were also able to obtain information on twinning rates from 1980-1985 in 112 countries.
They found substantial increases in twinning rates in many European countries, as well as in North America and in Asia.
Out of the 112 countries for which data were available for both study periods, there was an increase of 10% in 82 countries. There was a 32% increase in Asia and 71% in North America. A decrease of more than 10% was found in only seven countries.
Prof Monden said: “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.
“The twinning rate in Africa is so high because of the high number of dizygotic twins born there – twins born from two separate eggs. This is most likely to be due to genetic differences between the African population and other populations.
“The absolute number of twin deliveries has increased everywhere except in South America. 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.”
Most of the increase in twinning rates comes from dizygotic twins, while the rate of monozygotic twins has remained stable at about four per 1000 deliveries worldwide.
Prof Smits said the study has important implications, particularly because of the growth in twinning rates in low and middle income countries.
“Because infant mortality rates among twins have been going down, many more of the twins born in the second period of our study will grow up as twins compared to those born in the early ‘80s,” he said.
“However, more attention needs to be paid to the fate of twins in low and middle income countries. 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. While twinning rates in many rich Western countries are now getting close to those in sub-Saharan Africa, there is a huge difference in the survival chances.”
The researchers say countries, particularly low-income ones, must keep better data on twin births, as reliable statistics are lacking in many places. This is the main limitation of the study.
They add they will update their results with data for the early 2020s to see what has happened to twinning rates.
Monden C, Pison G, Smits J. Twin Peaks: more twinning in humans than ever before. Human Reproduction 12 March 2021
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