More human twins are being born than ever before, largely because of an increase in IVF and other techniques and a rise in average age of mothers.
According to the first comprehensive global overview, published in Human Reproduction, since the 1980s the twinning rate has increased by a third from 9 to 12 per 1000 deliveries. This means that about 1.6 million twins are born each year worldwide. One in every 42 children born nowadays is a twin.
Professor Christiaan Monden, the first author of the study, says: "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."
A major cause of this increase is the growth in medically assisted reproduction (MAR), which includes not only IVF techniques, but also ovarian stimulation and artificial insemination. Another cause is delay in childbearing since the twinning rate increases with the mother's age.
We may have reached the peak in twinning rates, particularly in high income countries such as Europe and North America, because of increasing emphasis amongst fertility doctors on the importance of trying to achieve singleton pregnancies.
Prof Monden and his colleagues 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.
They found substantial increases in twinning rates in many European countries, in North America and in Asia. For 74% out of the 112 countries the increase was more than 10%. There was a 32% increase in Asia and a 71% increase in North America. A decrease of more than 10% was found in only seven countries.
Prof Monden said: "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 there has been little change in the rate of monozygotic twins (twins from the same egg), which has remained stable at about 4 per 1000 deliveries worldwide.
"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. 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."
Michael Cook is editor of BioEdge
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