Length of gestation and birthweight in dizygotic twins

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 I nvestigate the Influence of sex on gestation, we compared birthweight and g estation of 1929 same-sex and unlike-sex dizygotic pairs. Length of gestati on In unlike-sex pairs was similar to that of female same-sex pairs, and si gnificantly (0.4 weeks; p=0.02) longer than that of male same-sex pairs. Bi rthweight 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 palm It I s the girl that prolongs gestation for her brother, resulting In a higher b irthweight than that of same-sex boys.