Haemodynamic model of twin-twin transfusion syndrome in monochorionic twin pregnancies.

Twin-twin transfusion syndrome in monochorionic twin pregnancies is not understood completely and is controversial which hampers development of acceptable diagnostic and rational treatment strategies. A haemodynamic model was developed that relates fetal growth with (1) fetoplacental blood flow and fetomaternal effects, and (2) net twin-twin transfusion from donor to recipient twin. Fluid balance mechanisms were neglected. Placental vascular anastomoses (arteriovenous, venoarterial, arterioarterial, venovenous) were modelled as straight blood vessels connecting the placental cord insertions that grow during pregnancy. Poiseuille's law predicts significantly decreasing anastomosing resistances, and when placental sharing is unequal it is assumed that smaller placental fractions cause smaller blood volumes and pressures. Two coupled first-order differential equations describing each twin's blood volume were determined and analysis showed that placental and anastomotic development cause anastomotic blood flow to increase faster than fetal growth. Hence, it is proposed as the syndrome's underlying pathophysiology that fetal discordance increases progressively, beyond fetal compensatory capacity. Fewer anastomoses cause larger discordance, but its onset can vary widely during pregnancy. Arteriovenous plus compensating anastomoses produce dynamic steady-state growth patterns with large, opposite, measurable anastomotic blood flows. Clinical study of fetal growth patterns may identify the syndrome's underlying placental anatomy. Predicted trends depend only weakly on implemented fetal physiology and are most likely realistic. This knowledge could improve future management of the syndrome.