Polyhydramnios and arterio-arterial placental anastomoses may beneficiallyaffect monochorionic twin pregnancies

Our objective was to appraise whether an increased amniotic fluid pressure by polyhydramnios can beneficially affect monochorionic twins that are haem odynamically connected by arterio-venous plus arterio-arterial placental an astomoses. We assessed the effects of polyhydramnios in monochorionic twin placentas, combining (a) data from previous in vitro placental perfusion ex periments in singleton tel-nl placentas under simulated normal and increase d amniotic fluid pressures with (b) logical deduction from observations mad e in monochorionic twins. Our hypothesis is that in monochorionic placentas , an increased amniotic fluid pressure increases the placental microvascula r resistance but not the resistance of placental chorionic plate arteries. Hence, an increased amniotic fluid pressure increases the microvascular res istance of the joint cotyledon, the anterio-venous resistance, but not the arterio-arterial resistance, This proposed mechanism reduces arterio-venous but not oppositely directed arterio-arterial transfusion. Therefore, rever sal of the normal direction of net foeto-foetal transfusion may develop, wh ich will reduce the circulatory imbalance that evolved between the monochor ionic foetal twins. In contrast, in monochorionic twins connected by unidir ectional or bidirectional arterio-venous anastomoses reversal of the normal direction of net foeto-foetal transfusion will not occur. In conclusion, r eversal of the normal direction of net foeto-foetal transfusion, induced by polyhydramnios, is protective against the onset and severity of twin-twin transfusion syndrome in monochorionic twins connected by arterio-venous plu s arterio-arterial anastomoses, but not by unidirectional or bidirectional arterio-venous anastomoses.