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The mothers of the twins are no more prolific, just lucky – ScienceDaily

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Or more fertile women who have twins? While previous research has concluded that this is the case, a detailed analysis of more than 100,000 births from pre-industrial Europe by an international team of scientists shows that this is not the case. The results of the study are now published in a scientific journal The nature of communication.

In humans, twins are common in about 1-3% of all births. Gemini is found in all populations, although it is associated with a much higher risk of natal and postnatal health problems for both mother and her children than singleton pregnancies. Given these risks, it seems that natural selection prevented nearby in the course of evolution. But why then has evolution by natural selection not prevented twinning?

One common explanation was that the risks to survival associated with twinning are partly hidden from natural selection because twinning brings higher birth rates. The idea is that women with more fertility than the average are more likely to release more than one egg during ovulation, making twins a marker of high fertility. Many studies have analyzed demographic data and obtained results that are consistent with this view.

However, this new study shows that previous analyzes were erroneous. “Previous research is problematic because it cannot tell us whether mothers give birth to twins more often because they are particularly fertile, or because birth more often increases the likelihood that one of these births will lead to twins.” explains lead researcher Alexander Curtiol of the Leibniz Institute for Zoos and Wildlife Research in Germany.

New results show that the twins are not particularly fertile. Previous science mixed cause and effect. “If the mother gives birth more often, it is more likely that one of these births is due to twins – just as you are more likely to win if you buy more lottery tickets, or get into a car accident if you drive a lot,” adds first author Ian Ricard of the University of Durham, UK, which, given the “lottery ticket effect”, found that mothers are more likely to give birth to twins less often – a result that contradicts previous findings.

To reconsider the link between twinning and fertility, an international team of 14 scientists pooled large datasets on birth outcomes from several parts of pre-industrial Europe (present-day Finland, Sweden, Norway, Germany and Switzerland). “All of this data comes from old parish records that have been carefully digitized and rewritten,” explains co-author Virpi Lumaa of the University of Turku, Finland. “To avoid the statistical trap that has plagued previous research, we have also had to deploy effective and carefully calibrated statistical procedures,” adds co-author Francois Rousseau of 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. Indeed, biomedical studies looking for ways to improve female fertility have compared mothers with and without twins. However, co-author Eric Postma of the University of Exeter in the UK notes that “such studies ignore many factors that affect how often a woman gives birth, masking any real differences in physiology between mothers with twins and without them. ». In short, comparing groups of mothers with twins with groups of mothers without them can hide the effects of twins and fertility genes where they exist, or create the illusion of them when they do not exist.

“We still don’t understand much about twins, but our research suggests that twins were not eliminated by natural selection for two reasons. First, twins are the result of double ovulation, which compensates for reproductive aging and benefits everyone except Secondly, if the risk of early mortality of twins is not too high, the twins are associated with a large family size, although women with twins give birth less often. This is because twins give birth to two offspring instead of one. ” – concludes Curtiol.

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