title: Size-discordant twin pairs have higher perinatal mortality rates than nondiscordant pairs.
abstract: OBJECTIVE
  The purpose of this study was to determine whether size-discordant twin pairs have worse perinatal mortality and neonatal morbidity rates than nondiscordant pairs and whether the smaller twins of discordant pairs have worse perinatal outcomes than the larger twins.
  
  
  METHODS
  We conducted a population-based, retrospective analysis of linked birth certificates and fetal and infant death certificates for 9590 twin pairs who were born in the state of Washington from 1987 through 1999. The Cochran-Mantel-Haenszel test, Student t test, and McNemar test were among the tests used to assess statistical significance.
  
  
  RESULTS
  Discordant twin pairs had higher rates of perinatal mortality, neonatal mortality, and 5-minute Apgar scores of <7, even after stratification by gestational age. Discordant pairs had lower pair weights at each gestational age and were more likely to include small-for-gestational-age infants. Compared with the larger twins, the smaller twins of discordant pairs had higher rates of perinatal mortality.
  
  
  CONCLUSIONS
  Discordant pairs had worse perinatal outcomes within each gestational age category.
authors.affiliation: Department of Biology, Seattle University, WA 98122, USA.
authors.forename: Rebecca S
authors.initials: RS
authors.lastname: Hartley
authors.forename: Jane
authors.initials: J
authors.lastname: Hitti
authors.forename: Irvin
authors.initials: I
authors.lastname: Emanuel
mesh_terms: D000328:Adult; D001724:Birth Weight; D001824:Body Constitution; D005260:Female; D005865:Gestational Age; D006801:Humans; D007226:Infant Mortality; D007231:Infant, Newborn; D007236:Infant, Small for Gestational Age; D011247:Pregnancy; D014427:Twins