Monoamniotic-versus diamniotic-monochorionic twin placentas: Anastomoses and twin-twin transfusion syndrome

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. STUDY DESIGN: 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). CONCLUSION: 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