title: Monoamniotic-versus diamniotic-monochorionic twin placentas: anastomoses and twin-twin transfusion syndrome.
abstract: OBJECTIVE
  The purpose of this study was to compare monoamniotic-monochorionic and diamniotic-monochorionic twin placentas and to estimate the incidence of twin-twin transfusion syndrome in monoamniotic-monochorionic twin pregnancies.
  
  
  METHODS
  We analyzed the angioarchitecture and cord insertion distance in 24 monoamniotic-monochorionic and 200 diamniotic-monochorionic placentas.
  
  
  RESULTS
  Compared with diamniotic-monochorionic placentas, monoamniotic-monochorionic placentas had significantly more arterioarterial anastomoses (20/20 vs 159/200, respectively; P=.013), significantly less opposite arteriovenous anastomoses (10/20 vs 165/200, respectively; P=.002), similar venovenous anastomoses (6/20 vs 46/200, respectively; P=.323), and arteriovenous anastomoses (20/20 vs 187/200 respectively; P=.279) and significantly shorter umbilical cord distances (median [+/-SD], 5.0+/-6.9 cm vs 17.5+/-6.8 cm; P<.001).
  
  
  CONCLUSIONS
  Monoamniotic-monochorionic and diamniotic-monochorionic placentas have different anastomotic patterns. The (virtually) 100% incidence of arterioarterial anastomoses in monoamniotic-monochorionic placentas explains the reason that twin-twin transfusion syndrome rarely occurs in monoamniotic-monochorionic twin pregnancies and predicts that twin-twin transfusion syndrome manifestations are approximately 5 times less often recognized in monoamniotic-monochorionic pregnancies than in diamniotic-monochorionic pregnancies.
authors.affiliation: Laser Center and Department of Obstetrics and Gynecology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands.
authors.forename: Asli
authors.initials: A
authors.lastname: Umur
authors.forename: Martin J C
authors.initials: MJ
authors.lastname: van Gemert
authors.forename: Peter G J
authors.initials: PG
authors.lastname: Nikkels
mesh_terms: D000650:Amnion; D001158:Arteries / Q000002:abnormalities; D001163:Arteriovenous Anastomosis / Q000473:pathology; D005260:Female; D005330:Fetofetal Transfusion / Q000150:complications* / Q000453:epidemiology / Q000473:pathology; D006801:Humans; D015994:Incidence; D010920:Placenta / Q000098:blood supply*; D011247:Pregnancy; D011272:Pregnancy, Multiple; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic; D014680:Veins / Q000002:abnormalities