title: Does chorionicity or zygosity predict adverse perinatal outcomes in twins?
abstract: OBJECTIVE
  The purpose of this study was to evaluate chorionicity and zygosity as risk factors for adverse perinatal outcomes in twins.
  
  
  METHODS
  A population-based, retrospective cohort study was conducted of all twin deliveries in Nova Scotia, Canada, from 1988 to 1997. Chorionicity was established by histologic examination. Zygosity was determined by chorionicity, sex, and infant blood group. Three groups were established: monochorionic/monozygotic twins, dichorionic/dizygotic twins, and dichorionic/majority monozygotic twins.
  
  
  RESULTS
  Outcomes from 1008 twin pregnancies were analyzed. Monochorionic/monozygotic twins had lower mean birth weights compared with dichorionic/dizygotic twins. Rates of perinatal mortality of at least 1 twin were significantly higher among monochorionic/monozygotic twins relative to dichorionic/dizygotic twins (relative risk, 2.5; 95% CI, 1.1-2.5). Dichorionic/majority monozygotic twins had similar perinatal outcomes compared with dichorionic/dizygotic twins.
  
  
  CONCLUSIONS
  Monochorionicity increases the risk of adverse perinatal outcome, whereas the effect of zygosity is less clear. Because chorionicity can be determined by prenatal ultrasound scanning, this information should be considered in the prenatal care of twin pregnancies.
authors.affiliation: Department of Obstetrics and Gynecology, Dalhousie University, Halifax, Nova Scotia, Canada.
authors.forename: Johanne
authors.initials: J
authors.lastname: Dubé
authors.forename: Linda
authors.initials: L
authors.lastname: Dodds
authors.forename: B Anthony
authors.initials: BA
authors.lastname: Armson
mesh_terms: D000328:Adult; D001724:Birth Weight; D002823:Chorion / Q000033:anatomy & histology* / Q000502:physiology*; D015331:Cohort Studies; D004200:Diseases in Twins / Q000209:etiology*; D005260:Female; D005317:Fetal Growth Retardation / Q000209:etiology; D005544:Forecasting; D006801:Humans; D007226:Infant Mortality; D007231:Infant, Newborn; D011247:Pregnancy; D011256:Pregnancy Outcome; D011272:Pregnancy, Multiple; D012189:Retrospective Studies; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic