Intertwin imbalance of amniotic fluid aldosterone levels in monochorionic twins: insight into twin-twin transfusion syndrome

The up-regulated renin-angiotensin-aldosterone system (RAAS) and the presence of placental anastomoses are assumed to cause twin–twin transfusion syndrome (TTTS) in monochorionic twins. The purpose of this study was to investigate the association between the presence of placental anastomoses and aldosterone (ALD) levels in amniotic fluid. This prospective study included monochorionic-diamniotic (MD) twins where amniotic fluid aldosterone levels were acquired from 2015 to 2021. Amniotic fluid samples were obtained from each fetal amniotic cavity at deliveries. Intertwin differences in amniotic fluid ALD levels (afALD) were examined in MD twins with placental anastomoses and those without placental anastomoses after fetoscopic laser surgery. The correlation of ALD levels between amniotic fluid and umbilical venous blood was also analysed. In 131 MD twins, the afALD level showed a significantly strong positive correlation with the umbilical venous ALD level (rs = 0.804, p < 0.001). In 41 MD twin pairs with placental anastomoses, the afALD level in smaller twins was significantly higher than wherein larger twins (p = 0.003). In 28 MD twin pairs without placental anastomoses, there was no significant intertwin difference between larger and smaller twins (p > 0.05). The imbalance of afALD levels between twins was observed in MD twin pairs with placental anastomoses. The results of this study provide insight that the presence of anastomoses might cause the intertwin imbalance of RAAS followed by the development of TTTS. In addition, afALD levels reflected ALD levels in umbilical venous blood and it seemed that afALD level has a possibility of clinical use for TTTS management.