Why are more and more twins being born?

Since the 1980s, the number of twins has increased by a third.

More twins are being born now than ever before - mainly due to the growing use of artificial insemination, as more and more people are opting for a family later in life. The birth rate of twins is thus perhaps the highest right now because of many infertility clinics. The number of twins has increased by a third since the 1980s, from nine children per thousand births to twelve per thousand children born. Christiaan Monden from the University of Oxford and his colleagues collected data in 165 countries between 1980 and 2015. Today, twins also have more chances of survival due to advanced medical technology. “The survival rate is higher than ever in the last 50 years,” Monden says. Most are fraternal or non-identical twins that develop from separate eggs and sperm. The rate of identical twins caused by splitting of the embryo in the first days after fertilization remained about the same.

Increasingly frequent infertility treatment is certainly the biggest factor in the rise in twin births in high- and middle-income countries, Monden explains. Women can take hormones to stimulate egg production, so they can release two eggs at the same time. In vitro fertilization clinics can also transfer two, three or more embryos to the uterus at the same time to increase the chances of at least one surviving. This can lead to pregnancy of twins or even triplets.

As the risk of child health is higher in such pregnancies, such as premature birth and low birth weight, many regulatory guidelines, such as the United Kingdom, state that fertility clinics should strive to transfer only one fetus. This, in turn, could mean that the birth rate of twins will start to decline.

Another factor contributing to the rise in twin births over the past three decades is women’s later decision to have a child, especially in high- and middle-income countries. Population growth has also been recorded in Africa, which has long had a relatively high rate of fraternal twins. In the 1980s, the birth rate of twins in sub-Saharan Africa was about twice as high as in the United Kingdom, although the United Kingdom has since caught up with Africa. The higher birth rate of twins in Africa is thought to be due to unknown genetic factors.

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