It has been postulated that male twin pregnancies, in contrast to male singleton pregnancies, differ in some distinctive biologic sense, leading to a shorter gestational duration and a lower individual birth weight than is the case in female twin pregnancies. To test this hypothesis in a relatively large dataset, information on gestational duration and birth weight for nearly all twins born in Sweden during a 4-year period (n = 3,472 twin pairs) was collected from the Medical Birth Registry, National Board of Health and Welfare. Included in the Medical Birth Registry are all pregnancies with a duration of at least 28 completed gestational weeks, or less if the newborns are alive at birth. Male-male pregnancies had a gestational duration similar to female-female pregnancies (median difference < 2 days). The proportions of twins with a gestational age < 36 weeks did not differ between male-male and female-female twin pregnancies (27.3 vs. 25.3%; χ2 0.05). Male-male pairs were heavier than female-female pairs (median difference 0.1 kg), and a significantly higher proportion of female-female twin pairs weighed < 2,500 g (45.0 vs. 39.2%; χ2 = 17.7, p < 0.001). The results of this study in an unselected relatively large twin population seem to indicate that fetal sex does not influence gestational duration to any significant extent. Males are heavier than females indicating that the sex has a similar effect on birth weight in twin and in singleton pregnanc