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Mothers of twins 'have heavier single babies' | Mothers of twins 'have heavier single babies' |
(about 3 hours 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 href="http://rsbl.royalsocietypublishing.org/lookup/doi/10.1098/rsbl.2011.0598" >report in the journal Biology Letters is based on a 40-year data set collected in The Gambia. | |
Mothers with twins are known to give birth to heavier babies, but the study found a similar trend even among single babies born before twins. | |
Twin pregnancies are risky for both mothers and offspring, and the study suggests heavier, healthier single children may offset those risks. | |
Worldwide only 13 in 1,000 babies are born a twin, although this rate is higher in developing countries. | 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. | 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 data set 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. | |
Harvesting data | |
The exact differences, however, depended on when those single babies were born. | |
The Gambia experiences regular variations in food supply, from a "harvest season" between January and June, and a "hungry season" for the rest of the year. | |
Single babies born during the harvest season before twins were on average 226g heavier than those from non-twinned families; those born after the twins were 134g heavier. | |
However, single children born into twinned families in the hungry season showed no discernible difference in average weight from those of non-twinned families. | |
"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. |
The assumption is that the stress of food scarcity swamps the heavier-baby effect found in the harvest season. | |
Producing twins, Dr Rickard suggested, could be just a by-product of natural selection acting on birth weight. | |
However, he 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. | He suspects that a hormone called IGF, which has long been linked to birth weight in humans, could be responsible for this pattern. |
IGF is known to influence the growth rate of foetuses, and is implicated in the "polyovulation" that leads to multiple births. | |
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. | 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. |