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Friday, 17 August, 2001, 00:13 GMT 01:13 UK
Girls make twin brothers heavier
A twin's weight may depend on the sex of their sibling
Boy twins are likely to be heavier at birth if the other twin is a girl, researchers have found.

Researchers from Belgium found that a woman carrying a female twin is also likely to have a longer pregnancy than one carrying two boys.

Our findings show that it is the girl who governs the length of gestation

Ruth Loos
They found that a female twin is highly likely to weigh less than her brother at birth.

The scientists, from the Centre for Human Genetics in Leuven, compared the birthweight and gestation period of 1,929 pairs of non-identical twins.

They found the gestation period for a pair of male twins was 0.4 weeks shorter than that for a pair of females, or an opposite-sex pair.

Female twins weighed the same, regardless of the sex of their sibling.

But male twins weighed on average 78g more if their fellow twin was female rather than male.


Lead researcher Dr Ruth Loos said: "Our findings show that it is the girl who governs the length of gestation.

"She prolongs gestation for her brother. He will benefit from the longer gestation, which results in a higher birthweight than that of male same-sex twins."

Dr Loos said the research suggested boy babies grow at a faster pace than girls, but girls catch up late in gestation.

This finding could be significant because fast growing foetuses are particularly vulnerable to adverse conditions in the womb, such as a lack of nutrition.

This might explain why some rates of disease are different among men and women in later life.

Dr Loos said she had no explanation for why girls prolong gestation.

She told BBC News Online: "Maybe in the case of female foetuses, the signal to deliver is postponed and laboured is delayed."

The research is published in The Lancet.

See also:

26 Jul 01 | Health
Folic acid 'doubles twin chance'
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