Twin twin transfusion syndrome (TTTS) is a rare condition seen in twin monochorionic pregnancies, typically developing during the 15-26 week gestation period and usually due to unbalanced intertwin placental anastomoses, where an unequal exchange of blood between twins causes oligohydramnios in one sac and polyhydramnios in the other which can lead to a high perinatal mortality rate and a high rate of disability in survivors if left untreated. The disease is twin to twin transfusion syndrome (MONDO_0019805). Also known as: Feto-fetal transfusion syndrome, Twin Anemia Polycythemia Sequence, Twin to twin transfusion, Twin-Twin transfusion syndrome, Twin-to-twin blood transfer, fetal blood loss from fetal hemorrhage into co-twin, fetal hemorrhage into co-twin, foetal blood loss from foetal haemorrhage into co-twin.