title: Birth weight discordancy and adverse perinatal outcomes among twin gestations in the United States: the effect of placental abruption.
abstract: OBJECTIVE
  We evaluated whether the relationship between birth weight discordancy of twins and stillbirth, neonatal deaths, and preterm births was modified by the presence of abruption.
  
  
  METHODS
  We used the 1995 to 1997 matched multiple birth file for United States twin births (n = 269287). Birth weight discordancy was defined as the ratio of the difference in birth weight of the heavier from the lighter twin to that of the heavier twin and was categorized as <5%, 5% to 9%, 10% to 14%, 15% to 19%, 20% to 29%, 30% to 39%, and >or=40%. We evaluated the risks of stillbirth (>or=20 weeks of gestation), neonatal deaths (within 28 days after birth), and preterm birth (< 32 weeks) in the presence and absence of abruption. Associations between birth weight discordancy and these perinatal outcomes were expressed as adjusted relative risks and were derived from multivariable logistic regression models, based on the method of generalized estimating equations. Risk of these outcomes were derived for each stratum of birth weight discordancy and abruption status, with the <5% birth weight discordancy, nonabruption status labeled as the reference group. All analyses were performed separately for same and different sex twins.
  
  
  RESULTS
  A birth weight discordancy of >or=20% among same sex (adjusted relative risk, 1.2; 95% CI, 1.1, 1.4), and >or=40% among different sex twins (relative risk, 2.2; 95% CI, 1.7, 2.8) conferred increased risk for abruption. Among nonabruption births, a birth weight discordancy of >or=15% among same sex and >or=30% among different sex twins increased the risk of stillbirths, neonatal deaths, and preterm births. Among abruption births, however, the risks were increased even in the lowest birth weight discordancy category (<5%). The relative risks of stillbirths and neonatal deaths among abruption births were significantly higher for each birth weight discordancy group, both for same and different sex twins, compared with the reference group. The association between birth weight discordancy and preterm birth was not modified by either the presence or absence of abruption.
  
  
  CONCLUSIONS
  Birth weight discordancy of >or=15% for same sex and >or=30% for different sex confer greatest risk of adverse perinatal outcomes in the absence of abruption. In the presence of placental abruption, these risks are further compounded. The results underscore the need for careful monitoring of twin pregnancies.
authors.affiliation: Section of Epidemiology and Biostatistics, University of Medicine and Dentistry of New Jersey, Robert Wood Johnson Medical School/Saint Peter's University Hospital, New Brunswick, USA.
authors.forename: Cande V
authors.initials: CV
authors.lastname: Ananth
authors.forename: Kitaw
authors.initials: K
authors.lastname: Demissie
authors.forename: Maryellen L
authors.initials: ML
authors.lastname: Hanley
mesh_terms: D000037:Abruptio Placentae / Q000453:epidemiology / Q000503:physiopathology*; D001724:Birth Weight; D005260:Female; D005313:Fetal Death / Q000453:epidemiology; D006801:Humans; D007226:Infant Mortality; D007231:Infant, Newborn; D007234:Infant, Premature; D008297:Male; D011247:Pregnancy; D011272:Pregnancy, Multiple; D015995:Prevalence; D012307:Risk Factors; D017678:Sex Distribution; D014427:Twins; D014481:United States / Q000453:epidemiology