By Mark Waghorn via SWNS
The idea that mothers of twins are more fertile is a myth, according to new research from Europe.
Having two babies at a time provides an evolutionary benefit, helping older women have bigger families from fewer pregnancies.
The study is based on more than 100,000 births from pre-industrial Europe - one of the biggest of its kind.
It found mothers more likely to have twins actually gave birth less often, a result that contradicts previous findings.
They are not unusually fruitful. An international team dubbed it the 'lottery ticket effect.'
Announcing the findings, they declared: "Mothers of twins are not more fertile, just lucky." Previous science had mixed up cause and effect.
First author Dr. Ian Rickard, of Durham University in the UK, explained: "If a mother gives birth more often, it is more likely that one of these births is to twins - just like you are more likely to win if you buy more lottery tickets, or to be in a car accident if you drive a lot."
In humans, twinning occurs in up to three percent of all births - despite a much higher risk of complications for both the mother and her children.
The prevailing theory is natural selection has prevented it from becoming more common. Yet evolution has not wiped it out altogether - creating a paradox.
One common explanation is women who are more fertile than average are also more likely to release more than one egg when they ovulate.
Many earlier studies analyzed demographic data and obtained results consistent with this view. But the latest demonstrates they were flawed.
Principal investigator Dr. Alexandre Courtiol, of the Leibniz Institute for Zoo and Wildlife Research, Germany, said: "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 is to twins."
By combining large historical datasets of birth outcomes from today's Finland, Sweden, Norway, Germany and Switzerland, the researchers found mothers of twins are not unusually fertile.
Co-author Professor Virpi Lummaa, of the University of Turku, Finland, said: "All this data originates from old parish records that have been meticulously digitized and transcribed."
While twinning can run in families, previous studies have suggested environmental factors are more important.
They include greater health in the mother or childbearing at later ages, which are more likely to produce twins.
Co-author Dr. Francois Rousset, of the University of Montpellier, France, said: "To avoid the statistical trap that plagued former studies, we also had to deploy efficient and carefully calibrated statistical procedures."
The study in Nature Communications has implications for public health as well as providing academic interest.
Biomedical studies looking for ways to improve female fertility have compared mothers with and without twins.
But co-author Dr. Erik Postma, of the University of Exeter, said: "Such study designs ignore the multitude of factors influencing how often a woman gives birth, which will mask any genuine differences in physiology between mothers with and without twins."
Comparing groups of mothers with and without twins may hide the effects of fertility genes where they exist or create the illusion of these if they do not exist.
Added Dr. Courtiol: "There is still much we do not understand about twinning, but our study suggests that twinning has not been eliminated by natural selection for two reasons.
"First, twinning is a consequence of double ovulation, which compensates for reproductive aging and benefits all but the youngest of mothers.
"Second, when the risk of early mortality of twins is not too high, twinning is associated with larger family sizes although women with twins give birth less often. This is because twin births bring two offspring rather than one."
A 2011 study by the University of Utah found women who deliver twins live longer and have more children than expected.
They also bear babies at shorter intervals over a longer time and are older at their last birth.
Senior author Prof Ken Smith said it means healthier women have an increased chance of delivering twins.
The number of twins has soared in recent decades since the introduction of assisted reproduction technologies.
The Human Fertilisation and Embryology Authority (HFEA) found just six percent of births from the treatment involved multiple children in 2019.
This was down from highs of 28 percent in the 1990s, with the huge plunge hailed by experts as being a "major success" for healthcare.
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