title: The Northwestern twin chorionicity study: testing the 'placental crowding' hypothesis.
abstract: OBJECTIVE
  To evaluate the relation between placental proximity and frequency of birth weight discordance and small-for-gestational age (SGA) infants.
  
  
  METHODS
  Retrospective three-tier chorionicity analysis of 1155 twin placentas comparing birth weight characteristics of the twins in different placental types.
  
  
  RESULTS
  Dichorionic-separate, but not dichorionic-fused twins, are heavier than monochorionic-diamniotic and monoamniotic twins (2376+/-721 vs. 2274+/-770, P < 0.006, and 2376+/-721 vs. 2166+/-782, P < 0.04). SGA twins are less frequent among dichorionic twins (OR 0.4; 95% CI 0.3, 0.6). Fewer sets with two SGA infants are present among dichorionic-separate compared to monochorionic-diamniotic pairs (OR 0.3; 95% CI 0.1, 0.8). The same trends are found when comparing all dichorionic to all monochorionic twins. Twins of all placental types have similar gestational ages and discordance values.
  
  
  CONCLUSIONS
  Dichorionic-separate placentas are least likely to experience 'placental crowding' and thus are associated with heavier twins and fewer sets with one or two SGA infants.
authors.affiliation: Kaplan Medical Center, Rehovot, Israel. blick@netvision.net.il
authors.forename: Isaac
authors.initials: I
authors.lastname: Blickstein
authors.forename: Saar
authors.initials: S
authors.lastname: Mincha
authors.forename: Ran
authors.initials: R
authors.lastname: D Goldman
authors.forename: Geoffry
authors.initials: G
authors.lastname: A Machin
authors.forename: Louis
authors.initials: L
authors.lastname: G Keith
mesh_terms: D000328:Adult; D001724:Birth Weight / Q000502:physiology; D005260:Female; D047109:Fetal Development / Q000502:physiology*; D005317:Fetal Growth Retardation / Q000503:physiopathology*; D006801:Humans; D010920:Placenta / Q000033:anatomy & histology* / Q000502:physiology; D010929:Placentation / Q000502:physiology; D011247:Pregnancy; D012189:Retrospective Studies; D014427:Twins / Q000502:physiology*