Why Humans Are Having More Twins Now Than at Any Time in History

The rate keeps rising for (naturally) two big reasons.

  • More twins are being born now than ever before. 
  • Parents are increasingly using in-vitro fertilization and deciding to have children later in life.
  • The twin rate could continue to go up as more nations push these factors higher.

The world is seeing double right now: Humans are having more twins at this moment in time than during any other point in recorded history, says a new study in the journal Human Reproduction.

How come? Credit a combination of different changes to family demographics over time, as well as factors like higher rates of in vitro fertilization (IVF) and an increase in older first-time parents.

In their study, the scientists, from Radboud University, University of Oxford, and the French Institute for Demographic Studies, found that one in 42 babies born today is a twin. That reflects an overall rate of 12 twin pregnancies out of 1,000, up from just nine in the 1980s. Twins are proportionately represented in the continents, so Asia and Africa, the two most populous continents, have 80 percent of total twin births around the world.

To analyze the combined rates of twins, the researchers studied two gigantic bodies of data. First, they pulled all available birth statistics from 112 countries from 1980-85, then compared the numbers with data from 2010-15, with 165 nations represented, “covering over 99 [percent] of the global population,” they say.

For 74 out of 112 countries, the increase in twins was more than 10 percent.

The biggest factor behind the spike is IVF and other medically assisted reproduction (MAR) procedures. 

“Large scale use of MAR started in the 1970s in the most developed countries, spread in the 1980s and 1990s to emerging economies in Asia and Latin America, and reached South Asia and the most wealthy groups in Africa only after 2000,” the researchers explain:

“We contrast twinning rates in 2010–2015, when the influence of MAR reached a peak, to rates in 1980-1985, when MAR was still at low levels, even in high income countries, and when genetic differences, overall fertility, age at childbearing and parity were the major driving factors. Since the early 1980s, many countries have seen significant changes in age at birth and parity distributions.”

In MAR, fertilizing and implanting multiple embryos boosts the chances for not just one, but multiple successful births.

The other major factor changing rates of twins is a rising parental age. The older the parent, the more likely they are to have naturally occurring multiple births, according to research from Brown University. That’s because older childbearing people have higher levels of follicle stimulation hormones (FSH), making it more likely to have multiple eggs released instead of just one. 

Will humans keep having even more twins? Maybe, because rates of use of MAR technologies vary around the world. In countries like Finland and New Zealand, for example, the effects of delayed parenthood are on the same scale as the effects of MAR. Meanwhile, in other countries like Spain and Singapore, the effect of MAR is five to six times higher, according to the study.

At the same time, places with lower use of birth control could see a reduction in twins as more people get access to better family planning. The scientists say there could also be other factors at play that we don’t know about yet. They also say their data isn’t perfect, and we should consider it a mere starting point for further investigation.

Source: Popular Mechanics