Objective: Twin studies offer opportunities to investigate mechanisms underlying sex-associated differences in perinatal outcomes. The objective of the study was to investigate sex-related differences in perinatal complications. Study design: A cohort of 16,045 twin pregnancies -32,090 twins -was explored for obstetric complications, perinatal and infant mortality, and neonatal morbidities. Results: Twin pregnancies with a female fetus had an increased risk for preeclampsia, but otherwise there were no pregnancy complications associated with fetal sex. After birth, female-female twins had lower early neonatal and infant mortality, and lower risk for respiratory morbidities than male-male twins at all gestational ages. In unlike-sexed twin pairs, very preterm males had higher respiratory morbidity than females and, females were at higher risk for being growth restricted. Conclusion: Male-male twins have higher respiratory morbidity and neonatal mortality than female-female twins. In unliked-sexed twin pairs, the males seem to be protected by having a female co-twin.

Dynamic BP responses differed between adolescents born VLBW-AGA and the other groups, indicating that extremely preterm birth may be a larger risk factor for increased BP than intrauterine growth restriction.