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 [CI] 1.2–2.6 and 4.1%, OR 2.3, 95% CI 1.3–4.2, respectively) and intermediate in the male–female group (7.5%, OR 1.4, 95% CI 1.1–1.9 and 3.2%, OR 1.8, 95% CI 1.2–3.0, respectively) using the female–female group as reference (5.5% and 1.8%, respectively). Male neonates in male–male 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.
LEVEL OF EVIDENCE: II