title: Perinatal mortality in first- and second-born twins in the United States.
abstract: OBJECTIVE
  To evaluate the prevailing mortality paradox that second-born twins are at higher risk of perinatal mortality than first-born twins.
  
  
  METHODS
  We used the 1995-1997 United States "matched multiple birth" data files assembled by the National Center for Health Statistics, for analysis of risk of perinatal mortality in first- and second-born twins (293788 fetuses). Perinatal mortality was defined to include stillbirths after 20 weeks of gestation and neonatal deaths (deaths within the first 28 days). Gestational age-specific risk of perinatal mortality (per 1000 total births), stillbirth (per 1000 total births), and neonatal mortality (per 1000 livebirths) by order of twin birth were based on the fetuses-at-risk approach. Associations between order of birth and mortality indices were evaluated by fitting multivariable logistic regression models based on the method of generalized estimating equations. These models were adjusted for several potential confounding factors.
  
  
  RESULTS
  Perinatal mortality was 37% higher in second-born (26.1 per 1000 total births) than in first-born (20.3 per 1000 total births) twins (adjusted relative risk [RR] 1.37; 95% confidence interval [CI] 1.32, 1.42). The increased risk of perinatal mortality in second-born twins was chiefly driven by a 2.46-fold (95% CI 2.29, 2.63) increase in the number of stillbirths. However, the risk of neonatal mortality was very similar between first- and second-born twins (RR 0.99, 95% CI 0.95, 1.04).
  
  
  CONCLUSIONS
  The increased risk of perinatal death in second-born twins is driven chiefly by increased rates of stillborn second twins. Thus, the increased mortality in second-born over first-born twins probably is an artifact of mortality comparisons.
authors.affiliation: Division of Epidemiology, School of Public Health, University of Medicine and Dentistry of New Jersey, New Brunswick, New Jersey 08901, USA.
authors.forename: Wendy
authors.initials: W
authors.lastname: Sheay
authors.forename: Cande V
authors.initials: CV
authors.lastname: Ananth
authors.forename: Wendy L
authors.initials: WL
authors.lastname: Kinzler
mesh_terms: D000328:Adult; D001722:Birth Order; D001724:Birth Weight; D015986:Confounding Factors, Epidemiologic; D005260:Female; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D016015:Logistic Models; D008499:Medical Records; D007744:Obstetric Labor Complications / Q000453:epidemiology*; D011247:Pregnancy; D011256:Pregnancy Outcome / Q000453:epidemiology*; D012189:Retrospective Studies; D012307:Risk Factors; D014427:Twins; D014481:United States / Q000453:epidemiology