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Mums Of Twins Birth Heavier Babes
Wednesday, 10 August 2011 03:58
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 study is based on a 40-year-dataset collected in Gambia, where seasonal food supply is variable.
The study is reported in reported in the journal Biology Letters.
Worldwide only 13 in 1,000 babies are born as one of a twin, although this rate is higher in developing countries.
When food is scarce - as it has been during much of human evolution - twin pregnancies are risky for both mums 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.
Bump-data
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.
Evolutionary biologist Ian Rickard from the University of Sheffield, UK, saw an opportunity to do just this when he learned of a long-term dataset from a rural 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.
Intriguingly, the difference in birth weight between twin-producing mothers and singleton-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.
A heavier hormone
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".
Dr Rickard 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.
Written by Jennifer Carpenter Science reporter, BBC News
Source: BBC News
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