title: Effect of fetal sex on pregnancy outcome in twin pregnancies.
abstract: 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.
  
  
  CONCLUSIONS
  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.
  
  
  METHODS
  II.
authors.affiliation: From the Helen Schneider Hospital for Women, Rabin Medical Center, Petach Tikva, and Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Nir
authors.initials: N
authors.lastname: Melamed
authors.forename: Yariv
authors.initials: Y
authors.lastname: Yogev
authors.forename: Marek
authors.initials: M
authors.lastname: Glezerman
mesh_terms: D000328:Adult; D001724:Birth Weight; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D007234:Infant, Premature; D007235:Infant, Premature, Diseases / Q000453:epidemiology; D020300:Intracranial Hemorrhages / Q000453:epidemiology; D008297:Male; D011247:Pregnancy; D011256:Pregnancy Outcome; D011272:Pregnancy, Multiple / Q000502:physiology*; D047928:Premature Birth / Q000453:epidemiology; D012127:Respiratory Distress Syndrome, Newborn / Q000453:epidemiology; D012307:Risk Factors; D012640:Seizures / Q000453:epidemiology; D012737:Sex Factors; D014427:Twins