© Getty Images

The twin boom era -

In 2024, the town of Inverclyde, Scotland, saw 10 sets of twins (six pictured) start elementary school all at once.

© Getty Images

Seeing double -

In the same month in the United States, 23 pairs of twins said goodbye to a Needham, Massachusetts, middle school as they finished their eighth grade studies. Earlier still in 2017, seven sets of twin siblings graduated from Portland High School in Maine.

© Getty Images

Two for a pair -

The past 30 years or so has seen an increase in the instance of twins, with 3.1% of births being twins in 2021, compared with 1.9% in 1980.

© Shutterstock

Double figures -

In fact, each year approximately 1.6 million twin pairs are born around the world. Back in the 1980s, this figure was a third lower.

© Shutterstock

Birthrate decline -

This is despite the fact that, globally, mothers are having fewer children. Yet while the birthrate appears to be in decline, the number of twins and triplets being born today is higher than ever before.

© Shutterstock

Assisted pregnancies -

There are several reasons why this may be the case. Primarily, it's because more people are seeking help with getting pregnant.

© Shutterstock

IVF treatment -

According to a report published by Ohio State University, medical reproductive technologies like in vitro fertilization (IVF) are one of the factors contributing to a rise in twin births.

© Shutterstock

Fertility treatments -

© Shutterstock

The donor insemination cycle -

According to the UK's Human Fertilisation & Embryology Authority (HFEA), more than 1.3 million IVF cycles and 260,000 donor insemination cycles have been carried out in Great Britain, leading to the birth of 390,000 babies.

© Shutterstock

Older-age pregnancy -

Interestingly, other factors believed to increase the odds of conceiving twins include older-age pregnancy.

© Shutterstock

Maternal age on the increase -

A 2022 report by the Associated Press (AP) revealed that the median age of US women giving birth is from 27 to 30, the highest on record.

© Shutterstock

Having children later -

In the United Kingdom, a survey conducted by the Office for National Statistics (ONS) found that, on average, most women in Britain now have their first child from the age of 32 onwards. In 1960, the median age was 26.

© Shutterstock

Risks of an older pregnancy -

As a woman gets older, the difficulty of getting pregnant and the percentage of miscarriages also increase.

© Shutterstock

Two by two -

But they are also likely to give birth to twins, even without using reproductive technologies like IVF. How is this so?

© Shutterstock

Fraternal twins -

Women in their thirties have a greater chance of conceiving fraternal twins as a result of an evolutionary response to combat declining embryo viability.

© Shutterstock

Two-child prospects -

In fact, a woman at 35 years old is four times more likely to have fraternal twins—a result from the fertilization of two separate eggs with two different sperm during the same pregnancy.

© Shutterstock

The ovulation process -

This occurs because it's more probable she'll release more than one egg during ovulation.

© Shutterstock

Running in the family -

The likelihood of conceiving twins is a complex trait and one that can also be decided by family history. Twins tend to be more common in certain families, a factor linked to the parent whose body releases the egg.

© Shutterstock

How genetics play a role -

For example, there may be an underlying genetic component that predisposes someone to release more than one egg at a time, ergo increasing the chance of having twins.

© Shutterstock

Ethnicity -

Race also plays a role in the twin boom. According to data published in 2024 by Statista, in the United States, non-Hispanic Black women currently have higher rates of twin births than any other ethnicity or race with 41.4 per 1,000 live births being twins.

© Shutterstock

Lowest rate of twin births -

Conversely, most statistics show that currently the lowest rate of twins is in the Asian population. Experts are at odds as to how much of a role ethnicity plays, but it may be due to dietary factors more than genetics.

© Shutterstock

Fraternal twins vs. identical twins -

It's worth pointing out the difference between fraternal twins and identical twins. As mentioned earlier, fraternal twins form from two eggs that are fertilized by two sperm.

© Shutterstock

Double take -

Identical twins form when one fertilized egg splits and develops two babies.

© Shutterstock

Hyperovulation -

Hyperovulation, also known as superovulation, is a rare condition that occurs when a woman releases more than one egg during a menstrual cycle.

© Shutterstock

Rare phenomenon -

This phenomenon is typically caused by fertility treatments or hormonal imbalances.

© Shutterstock

Multiple births -

Hyperovulation can increase the odds of conceiving twins or multiple pregnancies, for example triplets. Rarer still is the birth of four twins, or quadruplets. In extreme cases, quintuplets (five) and the multiple births of six twins have been recorded. In Morocco in 2021, a woman successfully gave birth to the world's first nonuplets, nine babies born at the same time and all apparently in good health.

© Getty Images

Global twinning rates -

In the wider world, twinning rates are estimated to increase in most low-income countries by 2050 and even more by 2100, according to the journal Human Reproduction.

© Getty Images

Twinning rates in low-income countries -

Many sub-Saharan African and South Asian countries are undergoing, and projected to further experience, the shift of maternal age at birth to older ages, the report notes. Presently, however, it's unknown how the changes in maternal age distribution are associated with the changes in twinning rates at the population level in these low-income nations.

© Getty Images

Is diet the reason? -

One of the highest rates of twinning in the world is to be found among a particular Indigenous group in Africa called the Yoruba. Interestingly, experts at Ohio State University cite diet as one possible reason the twinning rate is so high among the Yoruba. It's rich in a specific kind of yam that contains a phytoestrogen, or plant-like estrogen, that may increase the rate of multiple births.

© Getty Images

Highest twinning rate in the world -

But it's in the Ivory Coast where the highest rate of twinning has been recorded. According to a study of data from 135 countries published by the University of Oxford in 2020, the African country registered a rate of 24.9 twins for every 1,000 deliveries in the period of 2010–2015, a fact confirmed by Guinness World Records.  

Sources: (Geographical) (The Conversation) (Guinness World Records) (Ohio State University) (HFEA) (AP) (ONS) (Statista) (Parents) (Oxford Academic) (BBC)

See also: The countries with the highest fertility rates

© Getty Images