Giving birth to twins could be highly advantageous from an evolutionary perspective, according to Peeter Hõrak, professor of behavioral ecology at the University of Tartu. "This allows women to increase their reproductive rate. The younger and faster an organism reproduces, the greater the chance that its genetic variants will spread to subsequent generations," he explains.

However, it is well known that among all large mammals, from whales to great apes and humans, multiple births are very rare. Hõrak suggests that natural selection has historically acted against the birth of twins. "Since the likelihood of having fraternal twins is hereditary, natural selection can influence whether the birth of multiples becomes more common or rarer over the course of evolution," he says.

One of the factors through which natural selection operates is a woman's age. Together with animal ecology researcher Richard Meitern, Professor Allan Puur from the Demography Centre at Tallinn University, and researcher Mark Gortfelder, Hõrak sought to answer the question of whether the lifespan of women who gave birth to twins differs in any way from that of women who gave birth to single children. To do this, they examined family registry data from the years 1926-1943 in the Republic of Estonia. "We found that, in older age, mothers of twins had a greater chance of surviving beyond the age of 80," Hõrak notes.

Twins as a feat of childbirth

According to Peeter Hõrak, there are two hypotheses regarding the lifespan of women who give birth to twins. One is related to the so-called "cost of reproduction." "Even if a woman doesn't die in the year following the birth of twins, the physical demands associated with pregnancy and breastfeeding are much greater. Based on this, one could assume that she would have a shorter lifespan," Hõrak explains. This hypothesis has been confirmed in some studies based on data from the 18th and 19th centuries.

However, in their study, Hõrak and his colleagues tested another hypothesis, known as the supermother hypothesis. According to this hypothesis, mothers of twins are not a random segment of the population but are instead more resilient than others. "This has previously been demonstrated only once, using 19th-century data from Utah, USA. There, mothers of twins lived longer beyond the age of 55 compared to women who had only single children," the professor notes.

Hõrak and his colleagues then examined Estonian data. The research team compared the lifespan of all women who gave birth to twins with that of women who gave birth to single children. "We observed that the average lifespan after surviving childbirth was indeed longer for mothers of twins," says Hõrak. However, this result was misleading because the sample included women who had given birth to only one child. "Such women typically have a shorter lifespan than others due to health or other issues," the professor explains.

Therefore, the research team conducted a new analysis, taking into account various factors such as the number of children a woman had, her birth year and the age at which she gave birth to her first child. "This time, we found that the average lifespan of mothers of twins and single children did not differ," Hõrak recalls. However, the supermother hypothesis was still validated: after the age of 80, mothers of twins had a higher likelihood of survival compared to others.

But how can the long life of "supermothers" be explained? Hõrak points to his team's previous research, which suggests that while mothers of many children have more potential caregivers in old age, this does not seem to have a significant impact. "Mothers with eight or nine children still had a shorter average lifespan compared to those who had two or three children," he notes.

According to him, the birth of twins is more likely a testament to the woman's own vitality, as twin pregnancies are much more likely to miscarry compared to single pregnancies. "Successfully bringing twins into the world is, in itself, a confirmation that the woman is healthier and stronger than others. These qualities persisted into old age," he points out.

Life was better on the coast

Researchers suggest that the confirmation of the supermother hypothesis reflects the relatively favorable living conditions in 19th-century Estonia. "The simplest explanation, both here and in Utah, is that the average lifespan of these women was relatively higher than in other populations, such as the Saami and Finns of the 18th and 19th centuries, where these factors were previously measured," notes Peeter Hõrak. In a population with a shorter average lifespan, the traits associated with being a supermother would not manifest.

Co-author Mark Gortfelder adds that much depends on what Estonia's 19th-century living conditions are compared to. "Compared to the Saami or contemporary Finland, our conditions were certainly better. However, from today's perspective, all aspects of living conditions were decidedly grim," he reflects.

The research team also observed that women born in cities during the second half of the 19th century had shorter lifespans than those born in rural areas. "Until the interwar period, urban mortality was higher than in rural areas because people lived more densely in cities. Infectious diseases, which were the predominant cause of death, spread more easily in cities and took their toll," Gortfelder explains.

The researchers also noted differences in the lifespans of women living on the coast versus inland. According to Hõrak, maternal mortality after childbirth was lower and life expectancy was higher in parishes along the coast. "By the second half of the 19th century, the passport law had already been passed, and peasants were free to move. It was the era of long-distance travel, with more trading activity in coastal villages," Gortfelder points out. Fishing provided better food security on the coast, contributing to better living conditions. However, the study did not find a higher incidence of twin births in coastal parishes compared to other areas.

Hõrak expresses his satisfaction as a researcher in confirming the supermother phenomenon using such a comprehensive dataset. "The family registry was much more extensive than most previous similar study datasets. This means that the data was of extremely high quality, allowing us to statistically control for all relevant factors," he describes.

According to Gortfelder, the family registry is somewhat akin to a historical population register, with some caveats. It is a retrospective dataset compiled during the Republic of Estonia, including elderly individuals at the time. "The prerequisite for inclusion in the dataset was that the person indeed lived during the Republic of Estonia, and all major demographic events for these individuals are recorded," says Gortfelder.

Today, it would not be possible to study mothers of twins or multiples in the same way. "With the use of modern fertility technologies, this new factor can significantly alter the picture," Gortfelder notes. Hõrak agrees, adding that women who have multiples with the help of fertility technology today are likely much more motivated to have children and may also be healthier and stronger themselves. "It's a significant difference that we were able to study mothers of twins in a population with natural fertility," he concludes.

Peeter Hõrak, Mark Gortfelder, and their colleagues have published their study in the journal Human Reproduction.

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