title: The association between fetal sex and preterm birth in twin pregnancies.
abstract: OBJECTIVE
  To assess the association between the fetal sex and preterm birth.
  
  
  METHODS
  We performed a retrospective population-based cohort study using the 1995-1997 registration twin data in the United States (148,234 live-birth twin pairs). The twin pairs were divided into 3 groups: male-male (male-male), female-female, and opposite sex. We used 3 different cutoff values of preterm birth: less than 28, 32, and 36 gestational weeks. The preterm birth rates among the 3 study groups were compared, and the adjusted risk ratios (relative risk) were estimated by multiple logistic regression.
  
  
  RESULTS
  The male-male twin pairs had the highest pre-term birth rate (less than 28 weeks: 4.9%; less than 32 weeks: 12.4%; less than 36 weeks: 40.2%), the female-female twin pairs were intermediate (less than 28 weeks: 4.1%; less than 32 weeks: 10.6%; less than 36 weeks: 37.8%), and the opposite-sex twin pairs had the lowest rate (less than 28 weeks: 4.1%; less than 32 weeks: 10.1%; less than 36 weeks: 36.8%). Adjustment for important confounding factors or excluding twin pairs born to mothers who had an induction of labor or a cesarean delivery with medical complications did not change the results. The adjusted relative risks (95% confidence intervals) were 1.19 (1.11, 1.27), 1.21 (1.16, 1.26), and 1.09 (1.07, 1.11), respectively, for male-male twins compared with the opposite-sex twins under the 3 different cutoff values of preterm births.
  
  
  CONCLUSIONS
  Male sex is associated with increased risk of preterm births in twin pregnancy.
  
  
  METHODS
  II-2
authors.affiliation: OMNI Research Group and Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Faculty of Medicine, University of Ottawa, 501 Smyth Road, Ottawa, Ontario, Canada K1H 8L6. swwen@ohri.ca
authors.forename: Hongzhuan
authors.initials: H
authors.lastname: Tan
authors.forename: Shi Wu
authors.initials: SW
authors.lastname: Wen
authors.forename: Mark
authors.initials: M
authors.lastname: Walker
authors.forename: Karen Fung Kee
authors.initials: KF
authors.lastname: Fung
authors.forename: Kitaw
authors.initials: K
authors.lastname: Demissie
authors.forename: George G
authors.initials: GG
authors.lastname: Rhoads
authors.forename: Walker
authors.initials: W
authors.lastname: Mark
mesh_terms: D015331:Cohort Studies; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D007234:Infant, Premature; D008297:Male; D007752:Obstetric Labor, Premature; D011237:Predictive Value of Tests; D011247:Pregnancy; D011272:Pregnancy, Multiple; D012189:Retrospective Studies; D018570:Risk Assessment; D012680:Sensitivity and Specificity; D012727:Sex Characteristics; D014427:Twins; D016216:Ultrasonography, Prenatal