According to a study conducted by Oxford University, the world now has more twins than it has ever had, reports the Guardian.

The study, which analysed records from more than 100 countries, points to the global twin birthrate having risen by one-third, on average, over the past 40 years, since the1980s.

However, the peculiar trend seems to have hit its peak, as records post-2019 show a sharp fall in the rate of twins born across the world.

Globally, twinning rates have gone from 9 to 12 per 1,000 births since the 1980s. But what’s interesting is that this trend is largely restricted to rich and developed countries.

According to a study conducted on the birth rate of twins in low and middle-income countries of the developing world, between the years 1987 and 2010, East and South Asian countries reported a very low rate (6 - 9%) of twin births, as did Latin America.

While Latin America continues to have a low rate, there has been a steady spike in the number of twins born in Asia since 2011, according to the new study.

Also, the twinning rate in Africa—which previously has a high rate of natural twin births, continued to be high.

The study’s head, however, attributes the growth rate in Africa to the general rise of population and a genetic disposition.

While on the other hand, the study found that the greatest rises were seen in North America (71 per cent), Europe (60 per cent).

The reason for this disparity may lie in the key reasons behind this phenomenon itself.

But first, a quick run-through of how twins are made.

There are two main ways that a multiple pregnancy can happen:

• One fertilized egg (ovum) splits before it implants in the uterine lining.
• Two or more separate eggs are fertilized by different sperm at the same time.

Both the cases can lead to either fraternal or identical twins.

The two main reasons for the rise in twins being born in the last few decades are the increase in medically assisted reproduction, and women choosing to get pregnant later in life.

According to WebMD, there are multiple reasons behind the direct correlation between the age of a woman when she gets pregnant and the likelihood of having twins.

Moreover, medically assisted reproduction, including IVF, ICSI, artificial insemination, and ovarian stimulation.

“It doesn’t surprise us that twinning rates have increased because the availability of assisted reproduction has increased and also because women are slightly older when they have their first children, and both those things will increase the twin rate,” says Raj Mathur, the chair of the British Fertility Society and a consultant gynaecologist at St Mary’s Hospital in Manchester.

For instance, in 2007, UK’s Human Fertilisation and Embryology Authority (HFEA) launched a campaign to reduce multiple births which has resulted in a year-on-year reduction in multiple birth rates, from a 20% to a 10%.

The medical fraternity prefers avoiding multiple births if possible, for the reason that while twins who make it can have just as long and healthy lives as their single children, twin pregnancies usually carry a higher risk for both mother and the babies.

(Written with inputs from the Guardian and BBC.)