OXFORD, England — The number of twins being born in the U.S. has increased by more than 70 percent over the past three decades, according to new research. Fertility treatments, older mothers, and the pill are fueling the phenomenon, say scientists.
Around 1.6 million twins are born worldwide every year. That means one in every 42 children born is a twin. The rate has risen a third since the 1980s from 9 to 12 per every 1,000 deliveries. Celebrity mothers of twins include Angelina Jolie, Julia Roberts, Beyonce, Jennifer Lopez, Celine Dion, and Mariah Carey.
A major cause for this is an increase in medically assisted reproduction (MAR), including in vitro fertilization (IVF), ovarian stimulation, and artificial insemination. Delayed childbearing, a trend seen in many countries, is another factor. The likelihood of having twins increases with a mother’s age.
“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,” notes study first author Professor Christiaan Monden, of the University of Oxford, England, in a statement. “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 study is the first comprehensive global analysis of its kind. It was based on data on twinning rates from 165 countries between 2010 to 2015, covering 99 percent of the world’s population. This data was compared to the same information the researchers were able to obtain for 112 countries from 1980 to 1985.
The increase was 71, 61, and 32 percent in the U.S., Europe, and Asia, respectively. Substantial increases were also identified across Europe. It was more than a ten percent increase for three in four countries (74%) covered by both periods. A decrease of more than ten percent was found in only seven countries.
“In both periods, Africa had the highest twinning rates and there was no significant increase over time,” says Prof Monden. “However, Europe, North America, and the Oceanic countries are catching up rapidly. About 80 percent 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 – 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,” he continues. “In North America and Africa, the numbers have increased by more than 80 percent, and in Africa this increase is almost entirely caused by population growth.”
What’s behind this great rise in twins?
There has been little change in the rate of monozygotic twins from the same egg: about four per 1,000 deliveries worldwide.
Fertility treatments started in wealthier countries in the 1970s. They spread to emerging economies in Asia and Latin America in the 1980s and 1990s and some parts of South Asia and Africa after the year 2000. Women choosing to start families later in life, increased use of contraception, and lower fertility overall also play a role.
But we may have reached the peak of twin births 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 Africa, is less certain. It has important implications for mothers and babies and also healthcare resources.
“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,” says study co-author Professor Jeroen Smits of Radboud University in The Netherlands. “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 plan to update their results with data for the early 2020s to see if twinning rates have started to decrease in high-income countries. They also want to discover the effect of the wider spread of MAR in low and middle-income countries.
This study is published in Human Reproduction.
SWNS writer Mark Waghorn contributed to this report.