More twins being born? Why studying twinning rate is important

Till now, most studies on twinning rates have focused on high-income countries; much less is known about the situation in low-income settings. This is exactly the gap a recent study by researchers from Max Planck Institute for Demographic Research, Germany and Swedish Collegium for Advanced Study tried to address.

They asked a specific question — “How are changing maternal age patterns and population growth expected to shape future twinning rates in low-income countries?” — to answer which, data from more than three million births across 39 low-income countries was analysed.

Using this data from two specific global surveys, Demographic Health Surveys (DHS) and World Fertility Surveys (WFS), the researchers looked at how women having children at older ages is linked to twin birth rates in low-income countries. Then, they used UN population projections to estimate how twin birth rates and the number of twin births might change over time.

The estimations for India are significant: even though the twin birth rate is expected to drop by about 10.5% by 2100, India’s large population means it will still have one of the highest numbers of twin births in the world.

What study found

Globally, the absolute number of twin births will rise in many countries due to population growth. That said, South Asia, specifically India and Bangladesh, are projected to see a decline in the number of twin births (10.5% for India, 20.2% for Bangladesh) because overall fertility is below replacement level and fewer women will be giving birth.

This decline is partially offset by more women having children at older ages, which increases the likelihood of twins. And despite this decline, India will still have the largest share of twin births (23.4%) among the studied countries due to its large population size.

Moreover, researchers flag that study’s projections for India may underestimate future twinning rates, because some important factors were not included.

The researchers first highlight that studies from high-income countries have identified the expansion of medically assisted reproduction (MAR) as a major reason for the rise in twinning rates. MAR includes treatments such as ovulation induction, artificial insemination, and Assisted Reproductive Technologies (ART) like in vitro fertilisation (IVF).

But in the data analysed for the present study, the researchers note that the uptake of MAR was low. They then specifically cite studies showing that the use of MAR in India has increased over the last two decades. They also refer to one study from several high-income countries which reported “an estimate of an average 40–50% increase in twinning rates if only the expansion of MAR had been in play, independently of delayed childbearing.” So India’s future twinning rates could be higher than what this study estimates, since MAR use in India is growing but wasn’t factored in.

Why this matters

The current study provides estimates and mainly highlights the need for greater attention in the future to support healthy outcomes for twin pregnancies and families with twins. Another important study explores broader trends related to twin births that carry significant public health implications.

A study published this year by researchers from Harvard Center for Population and Development Studies, Harvard T H Chan School of Public Health, Graduate School of Korea University and Institute of Economic Growth, India analysed the trends in twin births and deaths in India from 1993 to 2021. They specifically looked at the risk of death during different stages of early childhood, including the late newborn stage.

This study had five key findings. They are significant because it is probably the first analysis of twin mortality in India that utilises data from all five rounds of the National Family Health Survey (NFHS). First, the analysis showed rising twinning rates: the proportion of twin births in India increased from 0.9% in 1993 to 1.5% in 2021, in line with global trends.

Second, the analysis showed that although twins make up only a small share of total births, they accounted for 7.7% of all under-five deaths in India. Mortality among twins declined over time but remained very high (179.8 per 1000 live births in 2021).

Third, the analysis showed that twins faced much higher risks of dying compared to singletons—7.5 times higher in the early neonatal phase, and 10 times higher in the late neonatal phase.

Fourth, the analysis showed that twin survival varied sharply by household wealth – thereby highlighting the need to treat twins as a vulnerable group requiring targeted health interventions.

Fifth, twins are more often born to mothers in later pregnancies, but twins born in the first or second pregnancy are more likely to die.

On the basis of their analysis, the researchers specifically highlight the need to train frontline health workers to care for and monitor twins in order to improve birth outcomes at the local level. Most importantly, they emphasise the need to establish a twin registry. They point to twin registries in Australia, Denmark, Finland, and Sweden, which track health and development, provide economic insights, and support policymaking. These registries also help researchers understand the biological complexities of twinning and improve twin survival, the researchers highlight.

Understanding nature vs nurture

Twin registries are not only for improving twin survival. The data from twin research is invaluable for understanding how much diseases are influenced by genes versus the environment. By collecting and analysing DNA from twins, researchers can learn how genes and environment shape human traits and disease risks, which can guide accurate diagnosis, treatment, and prevention.

In 2022, researchers from AIIMS, ICMR, and the Mayo Clinic published a paper on the opportunities for establishing a twin registry in India. The paper, published in Twin Research and Human Genetics by Cambridge University Press, highlights experiences from countries that have already set up twin registries.

First, they describe the Swedish registry, which has 30 ongoing projects using their data to study public health issues such as cancer, dementia, cardiovascular disease, and allergies. They also highlight how the registry collaborates with molecular geneticists to identify disease-causing genes, and how a study of 400 Swedish twins is exploring the role of environmental factors in Parkinson’s disease. They also cite the Danish registry which is studying how different types of cancer are related, the risk factors for blood cancers, and how female sex hormones affect thinking and memory in middle-aged and older Danish twins.

Most importantly, they cite a study on how the Chinese National Twin Registry aims to address environmental exposures in the population. They argue that, since India has also seen a rise in urbanisation and pollution, twin studies investigating the incidence of asthma, chronic obstructive pulmonary disease, and other airborne inflammatory conditions would be of “immense value.” They point out that twins may respond differently to these environmental exposures depending on where they live. They also emphasise that a person’s immune system is shaped not only by their genes but also by their diet and gut microbiome.

Therefore, comparing twins who live in different conditions can reveal how genetics and the environment together influence the development and progression of these diseases.

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