The fact of giving birth to two babies, and not just one, which is the most common, could suggest that the mother is more fertile, which is why she has had a multiple pregnancy, however, an international team of 14 Scientists have now carried out a detailed analysis of more than 100,000 births that occurred in pre-industrial Europe and have concluded that these women are not actually more fertile, but, in any case, luckier for their double maternity.
Births of twins or twins constitute between 1 and 3% of all births, and in all populations there are twins, although pregnancies with more than one fetus are associated with a much higher risk of health problems, both for the future mother and for her children, compared to gestations of a single fetus. As a result of these risks it seems that natural selection has prevented fraternal twins from becoming more common during evolution. But in that case, why hasn’t evolution by natural selection completely prevented twinning?
One possible explanation for this would be that the survival risks associated with fraternal twins are partly hidden from natural selection because fraternal twins come with higher 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. In fact, many studies analyzed demographic data and their results were consistent with this view.
“Twins are a consequence of double ovulation, which compensates for reproductive aging and benefits all but the youngest mothers”
The new research – which has been published in the scientific journal Nature Communications – shows, however, that previous analyzes were flawed. “Previous studies are problematic because they cannot 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,” explains Alexandre. Courtiol of the Leibniz Institute for Zoo and Wildlife Research in Germany, and principal investigator.
Twins, female fertility and the effect of the lottery ticket
The results of the new work reveal 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 adds. first author Ian Rickard of Durham University, UK. Taking the ‘lottery ticket effect’ into account, the authors found that mothers more likely to have twins actually gave birth less often, contradicting previous findings
The researchers decided to re-examine the relationship between twinning and fertility by combining large datasets of births from various locations in 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,” says co-author Virpi Lummaa of 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,” notes co-author François Rousset of the Institut des Sciences de l’Evolution in Montpellier, France.
Discovering the relationship between twins and fertility is not only a topic of academic interest, but is also considered a public health issue, and biomedical studies aimed at finding ways to improve female fertility have compared to mothers with and without twins.
However, co-author Erik Postma of the University of Exeter in the UK points out that “such study designs ignore the multitude of factors that influence how often a woman gives birth, which will mask any genuine difference in physiology between mothers with and without twins”. “In short, comparing groups of mothers with twins to groups of mothers without twins can hide the effects of twinning and fertility genes where they exist, or create the illusion of them if they don’t exist.”
“There is still a lot we don’t understand about fraternal twins, but our study suggests that fraternal twins have not been eliminated by natural selection for two reasons. First of all, twins are a consequence of double ovulation, which compensates for reproductive aging and benefits all but the youngest mothers. Second, when the risk of premature twin mortality is not too high, twins are associated with larger families, although women with twins give birth less often. This is because twin births bring two children instead of one”, concludes Courtiol.
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