OBJECTIVE: To estimate the association between fetal sex and pregnancy outcome in dichorionic twin pregnancies and the effect of male and female fetuses on their opposite-sex co-twin. METHODS: This was a retrospective study of all dichorionic twin pregnancies at a tertiary hospital from 1995 to 2006. Pregnancies were classified into three groups according to fetal sex: female-female, male-female, and male-male. Pregnancy outcome was compared for the three groups. Neonatal outcome of female neonates from female-female pregnancies was compared with that of female neonates from male-female pregnancies. Similarly, the outcome of male neonates from male-female pregnancies was compared with that of male neonates from male-male pregnancies. RESULTS: Two thousand seven hundred four twin pregnancies were included in the study, of which there were 436 (16.1%) female-female pregnancies, 1,878 (69.5%) male-female pregnancies, and 390 (14.4%) male-male pregnancies. The risk of preterm delivery at less than 31 and 28 weeks of gestation was highest in the male-male group (9.2%, odds ratio [OR] 1.7, 95% confidence interval [Cl] 1.2-2.6 and 4.1%, OR 2.3, 95% Cl 1.3-4.2, respectively) and intermediate in the male-female group (7.5%, OR 1.4, 95% Cl 1.1-1.9 and 3.2%, OR 1.8, 95% Cl 1.2-3.0, respectively) using the female-female group as reference (5.5% and 1.8%, respectively). Male neonates in malemale twin pairs were characterized by a lower mean birth weight and a lower growth rate when compared with male neonates in male-female pairs. Female neonates from male-female pregnancies had a rate of respiratory and neurologic morbidity similar to that of male neonates and significantly higher than that of female neonates from female-female pregnancies. CONCLUSION: In twins, pregnancy outcome is enhanced when the fetus (male or female) shares the womb with a female rather than with a male co-twin. Analysis of neonatal outcome for preterm twin neonates identifies a male-offending factor.