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Updated:05/24/2022 17:23h
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Women who have twins are not more fertile. Although some previous studies had suggested it, a detailed analysis of more than 100,000 births carried out by an international team shows that they are not. The results are published in the journal Nature Communications.
In humans, twins typically occur in around 1-3% of all births. Twins occur in all populations, despite being associated with a much higher risk of natal and postnatal health problems for both the mother and her children than singleton pregnancies. Given these risks, it appears that natural selection has prevented twins from becoming more common during evolution.
But why then evolution by natural selection has not prevented it completely?
A common explanation has been that twin survival risks are partially hidden from natural selection because twins result from increased fertility. The idea is that women who are more fertile than average are also more likely to release more than one egg when they ovulate, making twins a marker of high fertility. Many studies analyzed demographic data and obtained results consistent with this view.
However, this new work shows that previous analyzes have been flawed. “Previous studies are equivocal because they can’t tell us whether mothers with twins give birth more often because they are especially fertile or because giving birth more often increases the chance that one of these births will be twins,” he explains. lead researcher Alexandre Courtiol of the Leibniz Institute for Zoo and Wildlife Research in Germany.
The new results indicate that the twins are not unusually fertile. Previous science had mixed cause and effect. “If a mother gives birth more often, one of these births is more likely to be twins, just as you are more likely to win if you buy more lottery tickets, or have a car accident if you drive a lot,” he explains. the first author Ian Rickard, from Durham University, United Kingdom.
When taking into account the “lottery ticket effect»the authors found that mothers more likely to have twins actually gave birth less often, a result that contradicts previous findings.
To re-examine the relationship between twinning and fertility, the international team of 14 scientists combined large datasets of births from various parts of the pre-industrial europe (present-day Finland, Sweden, Norway, Germany, and Switzerland).
“All of this data comes from old parish records that have been meticulously digitized and transcribed,” explains co-author Virpi Lummaa, from the University of Turku, Finland. “To avoid the statistical trap that plagued previous studies, we also had to implement efficient and carefully calibrated statistical procedures,” adds co-author Francois Rousset, from the Institute of Evolutionary Sciences in Montpellier, France.
Finding out what shapes the relationship between twins and fertility is not only a matter of academic interest, but also a matter of public health. In fact, biomedical studies looking at ways to improve female fertility have compared mothers with and without twins. However, co-author Erik Postma of the University of Exeter in the United Kingdom, notes that “such study designs ignore the multitude of factors that influence how often a woman gives birth, which will mask any genuine differences in physiology between mothers with and without twins.”
In short, he adds, “comparing groups of mothers with twins to groups of mothers without twins may hide the effects of twinning and fertility genes where they exist, or create the illusion of them if they don’t exist.”
“There’s still a lot we don’t understand about twins, but our study suggests that twins haven’t been eliminated by natural selection for two reasons. Firsttwins are a consequence of double ovulation, which compensates for reproductive aging and benefits all but the youngest mothers. Besides, when the risk of premature twin mortality is not too high, twins are associated with larger families, although women with twins give birth less frequently. This is because twin births bring two children instead of one.” concluye Courtiol.
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