For centuries, identical twins have fascinated parents and scientists alike. After all, what’s more amazing than two-of-a-kind human beings? Today, we’re living in a boomtime for twins. Since the 1980s, the number of twin births around the world has jumped by a third, and researchers are exploring the implications of this unprecedented surge.

There are serious health concerns to consider. Twins have more complications at birth, are more often born premature, and have lower birth weights and higher stillbirth and infant mortality rates. Risks for the mother — including gestational diabetes, pre-eclampsia and postpartum depression and maternal mortality — also substantially rise in twin pregnancies.

University of Oxford researchers compared births around the world between 1980 and 1985, and between 2010 and 2015. They found the number of twins jumped by a third, from nine out of 100,000 births to 12 out of 100,000.

They identified two main drivers. First is medically assisted reproduction techniques such as in vitro fertilization. Introduced in the 1970s, these techniques spread from wealthier countries to Asia and Latin America in the ’80s and ’90s. They reached South Asia and Africa around the year 2000.

A surge in twins then followed. Africa and Asia now account for 80% of all twin deliveries in the world.

The second factor is the older age of mothers giving birth, when twins are more likely.

Efforts are now underway in some countries to adjust medically assisted reproduction techniques to focus more on having successful single births, thus reducing the risks for both the baby and mother.

Time will tell if these steps will reverse the global twinning trend.