News Release

Girls make twin brothers heavier (p 560)

Peer-Reviewed Publication

The Lancet_DELETED

N.B If you are outside North America the embargo for Lancet press material is 0001 hours UK Time Friday 17th August 2001.

A Belgian study of non-identical twins published in a research letter in this week’s issue of THE LANCET shows that the female twin prolongs gestation resulting in a heavier male birthweight compared with birthweights of same-sex male twins.

Despite the longer gestation of girls, their birthweight is less than that of boys. Unlike-sex twins provide a natural situation which allows the study of the influence of sex on length of gestation, since both the male and female foetus share the womb. Ruth Loos and colleagues from the Centre for Human Genetics, Leuven, Belgium, compared birthweight and gestation of 1929 same-sex and unlike-sex dizygotic (ie. non-identical) twins to examine which sex of the unlike-sex twins determines the length of gestation, and consequently the birthweight of the co-twin.

Length of gestation in unlike-sex pairs was similar to that of female same-sex pairs, and 0.4 weeks longer than that of male same-sex pairs. Birthweight of female unlike-sex twins was similar to female same-sex twins, but male unlike-sex twins weighed on average 78 g more than male same-sex twins.

Ruth Loos comments: “Our findings in dizygotic twins confirm those in singletons-ie, despite the longer gestation, birthweight of female twins is lower than that of males. Most interesting are the unlike-sex pairs, showing 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. We do not know by which mechanism girls play this determining part, neither why unlike-sex boys weigh significantly more than same-sex boys, even after adjustment for gestation."

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Contact: Ms Ruth Loos, FLOK Katholieke Universiteit Leuven, Tervuursevest 101, B-3001 Heverlee, Belgium; T) 32-1633-2690 or 32-9228-4815; F) 32-1632-9197 or 32-9238-1823; E) ruth.loos@med.kuleuven.ac.be


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