Why more twins are being born than ever before – even though birthrates are falling

By Elizabeth Bailey for The Conversation

Birmingham, United Kingdom: Globally, mothers are having fewer children. Yet despite this latest observed decline in birth rates, the number of twins and triplets being born today is higher than ever before.

This is the first time this has ever happened in history – and research is predicting a continued rise in twinning rates. In previous years, twinning rates dropped in line with overall birth rates.

There are many reasons why this may be the case – and social drivers such as older-age pregnancy and greater use of fertility treatments appear to be key factors.

Although less common than pregnancy with one baby, multiple births are a natural part of human reproduction. About one in every 60 pregnancies is a multiple birth – whether that’s twins, triplets or even sextuplets.

Twins occur when two separate eggs are fertilised at the same time, or when a fertilised eggs splits into two. Multiple births can also happen as a result of “hyper-ovulation” – when more than one egg is released in the same cycle. This phenomenon is more likely to happen in women as they get older due to changes in the menstrual cycle’s hormonal patterns as they approach perimenopause.

Although very rare, hyper-ovulation may also lead to natural conception of what are known as “higher-order multiple pregnancies” – anything from triplets to nontuplets (nine babies).

Data from England and Wales in 2023 shows that for women under 20, one in 2,000 was a multiple birth – but for women aged 35-39, this rises to one in 57.

Recent research looking specifically at low-income countries has predicted a rise in multiple births across all countries from 2050 to 2100. The authors suggest these projections are driven by maternal age rises in these countries.

Birth trends

During the baby boom from the 1940s to 1960s, the number of multiple births in England and Wales was consistently around 12-13 per 1,000 pregnancies. Mothers in the 1960s were having their babies at an average age of 26 – an age when multiple birth is less likely.

But throughout the 1970s and 80s, with both the growing availability of family planning (including male and female sterilisation) and challenging economic times, there was a decline in larger family sizes. This gave rise to the stereotypical family size of 2.4 children.

Most women were having their babies in their mid- to late-20s during this period – around age 26 on average. This meant the UK multiple birth rate also dropped to its lowest – about ten per 1,000 maternities.

In the 1990s and 2000s, the multiple birth rate climbed in England and Wales. This was partly due to a slight rise in the average age that women first became mothers, but was largely driven by an increase in the use of fertility treatments.

In the early days of fertility treatment, it was common practice to place more than one embryo at a time to increase chances of pregnancy success. However, this also increased the chance of having a multiple birth. In the 1990s, the multiple pregnancy rate from fertility treatment rose as high as 28% (compared with a natural conception rate of 1 per cent - 2 per cent).

Concerns over the steep rise in multiple births due to fertility treatments, and the number of these babies being born premature as a result, led to the “One at a Time” campaign to promote only one embryo being transferred at a time. The rate of multiple births arising from fertility treatment in the UK is now only 4 per cent.

Yet from the mid-2000s to mid-2010s, multiple birth rates in the UK reached over 16 per 1,000 pregnancies (around 11,000 multiple pregnancies a year). This peak in multiple births is probably due to fertility rates rising in older women – but mainly the growing use of fertility treatments prior to the impact of the One at a Time campaign.

Following a dip through the 2010s, as the success of this campaign became apparent in the data, the UK multiple birth rate is now 14.4 per 1,000 maternities.

But more people are seeking fertility treatment now than ever before. In 1991, around 6,700 cycles of IVF were provided by fertility clinics in the UK. In comparison, 76,000 cycles of IVF were performed in 2021.

Fertility treatments can be expensive, and access to NHS-funded treatment is subject to eligibility restrictions in the UK, leading to more people considering seeking treatment abroad. Practices around multi-embryo transfer vary in European countries popular for fertility tourism from the UK – meaning the proportion of multiple pregnancies that occur from fertility treatment abroad could be much higher.

Having twins, triplets or more is undoubtedly a joy for many families – but it can come with additional hardships. In the UK, the stillbirth rate is almost double in twins compared with single pregnancies – and the rates of neonatal death in twins is more than three times higher. Pre-term birth happens in around 60 per cent of twins and nearly all triplets, quads or more. These infants often need neonatal care – leading to increased anxiety for parents.

Beyond birth, we know that the early years are very challenging – with additional support typically required for practical care such as feeding and sleep, alongside additional financial and emotional pressures. The financial cost to families of multiples is at least £20k more, compared with having two singletons in succession.

When planning for the shape of future maternity and early years care, the needs of the multiple birth population should be part of the conversation. It’s hard to predict how much rates could rise – especially as the figures are suggesting continued lower fertility rates overall. We may see fewer babies being born over time – but with an increase in the percentage of multiple births.

(Elizabeth Bailey is an Associate Professor and Director of the Elizabeth Bryan Multiple Births Centre, Birmingham City University.)