Assessing sex differences in neonatal survival: a study of discordant twins.

We identified 1699 liveborn twin pairs, discordant for sex. In this study, which essentially controls for gestational age, race, and maternal risk factors among males and females, there was no significant sex difference (108 male deaths and 103 female deaths) in neonatal mortality (p greater than 0.50). However, there was a sex difference in intrauterine growth, since 53% of the males, but only 42% of the females had birthweights greater than 2499 grams (p = 0.0002). A differential growth pattern can bias birthweight-specific assessments of survival. Such a bias may have been responsible for our finding that low-birthweight white females had better survival than did males in that category, since there was no such sex difference found among white twins born prematurely (greater than 36 weeks gestation). Therefore, we recommend that accurate assessments of sex differences in neonatal survival should be on the basis of gestational age, controlling for race and maternal risk factors.