The Northwestern University multihospital twin study: IV. Duration of gestation according to fetal sex

Recently MacGillivray has observed that the average duration of male twin gestations is shorter than that of either female twin or male/female twin gestations. The data from 562 twin gestations from the Northwestern University multihospital twin study were used to test this observation. Male twin gestations were slightly but significantly shorter than other twin gestations as measured by pediatric examination (p < 0.005). Approaching significance at the 0.05 level were findings that male twin gestations were shorter than other twin gestations when measured by dates, and that the male twins were slightly shorter and lighter than other twins. No significant differences were found between male/female twin gestations and female twin gestations. No relationship could be found between twin sex combinations and a variety of maternal and obstetric factors. Taken together, the results suggest that male twin gestations differ in some distinctive biologic sense from other twin gestations.