Objective: To assess the association between the fetal sex and preterm birth.
Methods: We performed a retrospective population-based cohort study using the 1995-1997 registration twin data in the United States (148,234 live-birth twin pairs). The twin pairs were divided into 3 groups: male-male (male-male), female-female, and opposite sex. We used 3 different cutoff values of preterm birth: less than 28, 32, and 36 gestational weeks. The preterm birth rates among the 3 study groups were compared, and the adjusted risk ratios (relative risk) were estimated by multiple logistic regression.
Results: The male-male twin pairs had the highest pre-term birth rate (less than 28 weeks: 4.9%; less than 32 weeks: 12.4%; less than 36 weeks: 40.2%), the female-female twin pairs were intermediate (less than 28 weeks: 4.1%; less than 32 weeks: 10.6%; less than 36 weeks: 37.8%), and the opposite-sex twin pairs had the lowest rate (less than 28 weeks: 4.1%; less than 32 weeks: 10.1%; less than 36 weeks: 36.8%). Adjustment for important confounding factors or excluding twin pairs born to mothers who had an induction of labor or a cesarean delivery with medical complications did not change the results. The adjusted relative risks (95% confidence intervals) were 1.19 (1.11, 1.27), 1.21 (1.16, 1.26), and 1.09 (1.07, 1.11), respectively, for male-male twins compared with the opposite-sex twins under the 3 different cutoff values of preterm births.
Conclusion: Male sex is associated with increased risk of preterm births in twin pregnancy.
Level of evidence: II-2