title: Twin chorionicity and the risk of stillbirth.
abstract: OBJECTIVE
  To estimate the effect of chorionicity on the risk of stillbirth in twins.
  
  
  METHODS
  A retrospective cohort analysis was performed of all twin deliveries of at least 24 weeks of gestation at a single tertiary care center from December 2000 to May 2007. The risk of fetal death with advancing gestation was calculated for monochorionic-diamniotic twins and for dichorionic-diamniotic twins. Overall in utero survival was compared by using Kaplan-Meier analysis and a hazards ratio with 95% confidence intervals estimated to assess the degree of difference. Pregnancies affected by growth abnormalities, anomalies, or twin-twin transfusion syndrome were subsequently excluded and survival by chorionicity similarly compared within these "apparently normal" gestations.
  
  
  RESULTS
  Data from 1,000 consecutive twin pairs (196 monochorionic-diamniotic twins and 804 dichorionic-diamniotic twins) were analyzed. Stillbirths occurred in seven (3.6%) monochorionic-diamniotic and nine (1.1%) dichorionic-diamniotic twin pairs. Monochorionic-diamniotic twins had a higher risk of stillbirth compared with dichorionic-diamniotic twins, both overall (log-rank P=.004) and at each gestational age after 24 weeks, with this risk persisting in the subset of 771 (130 monochorionic-diamniotic twins and 641 dichorionic-diamniotic twins) "apparently normal" twins (log-rank P=.039).
  
  
  CONCLUSIONS
  Monochorionicity has a negative effect on the in utero survival of twins, even among monochorionic-diamniotic twins without abnormalities.
  
  
  METHODS
  II.
authors.affiliation: Division of Maternal Fetal Medicine, Department of Obstetrics & Gynecology, Columbia University Medical Center, New York, New York 10032, USA. yml9002@med.cornell.edu
authors.forename: Young Mi
authors.initials: YM
authors.lastname: Lee
authors.forename: Blair J
authors.initials: BJ
authors.lastname: Wylie
authors.forename: Lynn L
authors.initials: LL
authors.lastname: Simpson
authors.forename: Mary E
authors.initials: ME
authors.lastname: D'Alton
mesh_terms: D000328:Adult; D000650:Amnion / Q000502:physiology*; D002823:Chorion / Q000502:physiology*; D015331:Cohort Studies; D016001:Confidence Intervals; D000013:Congenital Abnormalities / Q000453:epidemiology; D005260:Female; D005317:Fetal Growth Retardation / Q000453:epidemiology; D005330:Fetofetal Transfusion / Q000453:epidemiology; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D053208:Kaplan-Meier Estimate; D008297:Male; D016017:Odds Ratio; D011247:Pregnancy; D011248:Pregnancy Complications / Q000453:epidemiology*; D011256:Pregnancy Outcome; D012189:Retrospective Studies; D012307:Risk Factors; D050497:Stillbirth / Q000453:epidemiology*; D014427:Twins