Our objective was to appraise whether an increased amniotic fluid pressure by polyhydramnios can beneficially affect monochorionic twins that are haemodynamically connected by arterio-venous plus arterio-arterial placental anastomoses. We assessed the effects of polyhydramnios in monochorionic twin placentas, combining (a) data from previous in vitro placental perfusion experiments in singleton term placentas under simulated normal and increased amniotic fluid pressures with (b) logical deduction from observations made in monochorionic twins. Our hypothesis is that in monochorionic placentas, an increased amniotic fluid pressure increases the placental microvascular resistance but not the resistance of placental chorionic plate arteries. Hence, an increased amniotic fluid pressure increases the microvascular resistance of the joint cotyledon, the arterio-venous resistance, but not the arterioarterial resistance. This proposed mechanism reduces arterio-venous but not oppositely directed arterio-arterial transfusion. Therefore, reversal of the normal direction of net foeto-foetal transfusion may develop, which will reduce the circulatory imbalance that evolved between the monochorionic foetal twins. In contrast, in monochorionic twins connected by unidirectional or bidirectional arterio-venous anastomoses reversal of the normal direction of net foeto-foetal transfusion will not occur. In conclusion, reversal 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 plus arterio-arterial anastomoses, but not by unidirectional or bidirectional arterio-venous anastomoses.