Natural history of stage I disease

Twin-to-twin transfusion syndrome (TTTS) is a complication that affects approximately 10% of all monochorionic twin pregnancies. When the process begins in the second trimester and is left untreated, over 80% of affected pregnancies will end with the death of one or both fetuses, or with one fetus surviving with neurological deficit [1]. TTTS is classified based on the ultrasound findings [2]. Stage I is diagnosed when there is fluid discrepancy between the two fetuses with a donor sac with maximum vertical pocket (MVP) ≤2 cm and the recipient sac MVP of ≥8 cm; a visible donor bladder; and with normal Doppler flow studies in umbilical artery, umbilical vein, or ductus venosus of both fetuses. In European centers, after 20 weeks’ gestation the criteria for stage I diagnosis is recipient’s MVP of ≥10 cm. Stage II disease includes the findings of stage I along with an absent or non-cycling small donor bladder. Stage III disease is presence of abnormal Doppler flow studies in umbilical artery, umbilical vein, or ductus venosus in either fetus. Stage IV is when there is fetal hydrops in one of the fetuses. Stage V is when at least one of the fetuses is in demise. TTTS does not follow a sequential progression through the different stages of disease as it may skip stages within a defined time frame.