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Mothers of twins 'have heavier single babies' | |
(about 1 hour later) | |
By Jennifer Carpenter Science reporter, BBC News | By 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 leading to smaller, more vulnerable babies. | |
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. | |
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. | |
For a "twin gene" to exist, it must give its carrier an advantage, despite its riskiness - at least some of the time. | |
Food scarcity | Food scarcity |
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 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. | |
Producing twins, he suggested, 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". |
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 levels of the hormone are linked to larger litters. |