Mothers of twins 'have heavier single babies'
13 August BBC
Single babies born to mothers of twins tend to be heavier, report
scientists.
The 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.
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.
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.
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.
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