Twin-to-twin transfusion syndrome results from dynamic asymmetrical reduction in placental anastomoses: A hypothesis

Although placental vascular anastomoses between the fetoplacental circulati ons are ubiquitous in monochorionic twin pregnancies, the factors regulatin g their formation and maintenance are not understood. Increasing evidence i mplicates asymmetric anastomotic patterns in the aetiology of severe twin-t o-twin transfusion syndrome (TTTS). The authors propose that anastomoses be tween placental circulations in monochorionic ta ins occur in a random mann er at the embryological st-age of connection of embryonic and extra-embryon ic circulations. Placental expansion is then associated with random disrupt ion of anastomoses and regression of their associated villus districts. TTT S develops as discordant loss of anastomoses results in asymmetrical how re sistance. Pregnancies with fetal growth concordance but discordant nuchal t ranslucency at 10-14 weeks are at increased risk of developing subsequent s evere TTTS because these clinical features indicate significant pressure di fferentials in the presence of a placentoplacental circulation, consistent with the presence of numerous, asymmetric anastomoses. However, since the a nastomotic pattern is dynamic in the first half of pregnancy this hypothesi s predicts that it will not be possible to devise a clinical test at 12 wee ks that will predict with certainty the outcome of monochorionic twin pregn ancies in relation to TTTS because this depends on random subsequent events . (C) 2001 Harcourt Publishers Ltd.