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Mothers of twins have heavier single babies - research Mothers of twins 'have heavier single babies'
(about 1 hour later)
By Jennifer Carpenter Science reporter, BBC NewsBy Jennifer Carpenter Science reporter, BBC News
Single babies born to mothers of twins tend to be heavier, report scientists.Single babies born to mothers of twins tend to be heavier, report scientists.
The finding offers an explanation for why twinning exists despite it producing babies that are usually smaller and more vulnerable to disease and starvation. The finding offers an explanation for why twinning exists, despite it leading to smaller, more vulnerable babies.
The study is based on a 40-year dataset collected in The Gambia, where seasonal food supply is variable. The study, href="http://rsbl.royalsocietypublishing.org/lookup/doi/10.1098/rsbl.2011.0598" >reported in the journal Biology Letters, is based on a 40-year data set collected in The Gambia, where the seasonal food supply is variable.
The study is reported in the journal Biology Letters. When food is scarce, twin pregnancies are risky for both mothers and children - but the study addresses why, despite this, twins seem to run in families.
Worldwide only 13 in 1,000 babies are born a twin, although this rate is higher in developing countries. For a "twin gene" to exist, it must give its carrier an advantage, despite its riskiness - at least some of the time.
When food is scarce - as it has been during much of human evolution - twin pregnancies are risky for both mothers and their offspring.
And yet, twins seem to run in families.
For a "twin gene" to exist, it must give its carrier an advantage, despite its riskiness; at least sometimes.
Food scarcityFood scarcity
Researchers interested in probing the twinning question further have had to rely on the few long-term datasets collected in parts of the world where birth rates are higher, and there are therefore more twinning events. Worldwide only 13 in 1,000 babies are born a twin, although this rate is higher in developing countries.
Researchers interested in probing the twinning question further have had to rely on the few long-term data sets collected in parts of the world where birth rates are higher, and there are therefore more twinning events.
Evolutionary biologist Ian Rickard from the University of Sheffield in the UK, saw an opportunity to do just this when he learned of a long-term dataset from The Gambia, which included not only birth weights of about 1,900 babies born to around 700 mothers, but also the number of twins.Evolutionary biologist Ian Rickard from the University of Sheffield in the UK, saw an opportunity to do just this when he learned of a long-term dataset from The Gambia, which included not only birth weights of about 1,900 babies born to around 700 mothers, but also the number of twins.
Analysing all 40 years, Dr Rickard explained that he and his Gambian and London-based colleagues saw that women who produced twins gave birth to heavier non-twin babies - around 100g (0.2lbs) heavier, in fact.Analysing all 40 years, Dr Rickard explained that he and his Gambian and London-based colleagues saw that women who produced twins gave birth to heavier non-twin babies - around 100g (0.2lbs) heavier, in fact.
Intriguingly, the difference in birth weight between twin-producing mothers and singleton mothers disappeared when food was scarce. Intriguingly, the difference in birth weight between twin-producing mothers and single-baby mothers disappeared when food was scarce.
"We've known for quite a while that... if a [foetus] is exposed to a period of the year between about July and October during their third trimester they tend to have lower birth weight," said Dr Rickard. "We've known for quite a while that... if a [foetus] is exposed to a period of the year between about July and October during their third trimester, they tend to have lower birth weight," said Dr Rickard.
Cattle births Producing twins, he suggested, could be just a by-product of natural selection acting on birth weight.
Producing twins, said Dr Rickard, could be just a by-product of natural selection acting on birth weight.
However, Dr Rickard stressed the "importance of replicating this [finding] in another population to see if this pattern holds up".However, Dr Rickard stressed the "importance of replicating this [finding] in another population to see if this pattern holds up".
Dr Rickard suspects that a hormone called IGF, which has long been linked to birth weight in humans could be responsible for this pattern. He suspects that a hormone called IGF, which has long been linked to birth weight in humans, could be responsible for this pattern.
In cattle, IGF levels tend to be 1.5 times higher in the cows who give birth to twins, and in mice high hormone levels are linked to larger litters. In cattle, IGF levels tend to be 1.5 times higher in the cows who give birth to twins, and in mice high levels of the hormone are linked to larger litters.