Length Of Gestation

Length Of Gestation 

Background: Girls have both longer gestation and lower birth weights than boys. Unlike-sex twin births provide a natural situation to examine the influence of sex on gestation. The birth weight and gestation of same-sex dizygotic twins was compared with that of unlike-sex twins to examine the question of which sex determines the length of gestation.

Study Design: Data on 1929 dizygotic twin pairs were obtained from the East Flander Prospective Twin Survey.Birth weight and gestation were obtained from obstetric records. To determine which sex determines gestation length, the gestation periods of same-sex boys, same-sex girls, and unlike-sex pairs were compared.

Findings: The average gestation of unlike-sex pairs was similar to that of same-sex girls pairs but significantly longer than that of same-sex boy pairs. In line with this finding, the average birth weight 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 at birth.

Conclusions: These findings with dizygotic twins confirm that female babies have both a longer gestation and a lower birth weight. The findings with unlike-sex pairs indicate that the girl controls the length of gestation, prolonging her brother's gestation and causing him to have an increased birth weight. The mechanism by which female infants control gestation length is not known.