US researchers claim women who give birth to twins live longer than those who give birth to single babies. They speculate that the ability to successfully birth twins reflects a general biological robustness in the health of these women. A twin pregnancy is known to be more taxing on the mother's body and therefore was not thought to be biologically advantageous.
'The prevailing view is that the burden of childbearing on women is heavier when bearing twins. But we found the opposite: women who naturally bear twins in fact live longer and are actually more fertile', said study author Dr Shannon Robson from the University of Utah.
The population data used, from the US state of Utah, was collected between 1807 and 1899, before the advent of family planning techniques. This allowed 'natural twinning' to be studied without the results being skewed by medical interventions such as IVF.
The study focused on women born during this time window who had at least one child and lived until at least 50. But this did not take into account the increased chance of maternal death during the birth of twins.
'We do know women who have twins, triplets and so on do have medical complications and their health is sometimes compromised', said Dr Robson's co-author Professor Ken Smith, director of the Utah Population Database at the University of Utah. 'But we are talking about the long view'.
Of the approximately 60,000 women analysed, around 4,600 had twins. They were compared to the remaining women who had single babies.
Mothers who gave birth to twins were found to have more children in general, all of which were more likely to survive to maturity. These women also had shorter intervals between births and gave birth to their last child later in life. These results were more striking in mothers born before 1870, after which time modern medicine and family planning in Utah became more prominent.
Collectively these results point to mothers of twins being more fertile. Moreover, these mothers had increased lifespans. The authors therefore claim that the women's increased fertility was a sign of their general healthiness.
'There may be classes of women who have this capacity, that are particularly robust. There may be some constellation of traits that allows them to perform well, biologically', says Professor Smith.
Professor Smith went on to say that: 'It's not that you can go out and have twins in order to live longer. It's more that twinning is a reflection of an innate feature that we can't yet identify that leads to longer life…Twinning may be a part of the longevity recipe that people might want to know'.
This study was published in the Proceedings of the Royal Society B.