title: Length of gestation and birthweight in dizygotic twins.
abstract: Despite the longer gestation of girls, their birthweight is less than that of boys. Because unlike-sex twins provide a natural situation in which to investigate the influence of sex on gestation, we compared birthweight and gestation of 1929 same-sex and unlike-sex dizygotic pairs. Length of gestation in unlike-sex pairs was similar to that of female same-sex pairs, and significantly (0.4 weeks; p=0.02) longer than that of male same-sex pairs. Birthweight of girls from unlike-sex pairs was similar to that of girls from same-sex pairs, but boys from unlike-sex pairs weighed 78 g more than boys from same-sex pairs (p=0.001). These data show that in unlike-sex pairs it is the girl that prolongs gestation for her brother, resulting in a higher birthweight than that of same-sex boys.
authors.forename: R J
authors.initials: RJ
authors.lastname: Loos
authors.forename: C
authors.initials: C
authors.lastname: Derom
authors.forename: R
authors.initials: R
authors.lastname: Eeckels
authors.forename: R
authors.initials: R
authors.lastname: Derom
authors.forename: R
authors.initials: R
authors.lastname: Vlietinck
mesh_terms: D000704:Analysis of Variance; D001530:Belgium; D001724:Birth Weight; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D011446:Prospective Studies; D012737:Sex Factors; D014429:Twins, Dizygotic