Twin-twin transfusion syndrome: treatment and outcome.

Twin-twin transfusion syndrome complicates up to 15% of monochorionic pregnancies in the mid-trimester, and results in high perinatal mortality and morbidity if left untreated. Although some humoral factors play a role in the pathogenesis of the disease, an unequal placental sharing and the presence of placental vascular anastomoses at the chorionic plate, allowing blood volume shifts between the twins, are the anatomic prerequisite for this complication unique to monochorionic twins. Within monochorionic pregnancies, it is possible to identify a subgroup of twin pairs at high risk of developing twin-twin transfusion syndrome or selective intrauterine growth restriction during the course of pregnancy as early as in the first or early second trimester. If progressive amniotic fluid discrepancy advances to moderate twin-twin transfusion syndrome, and finally reaches the stages of severe twin-twin transfusion syndrome, accurate classification of the clinical picture and diagnosis of the individual fetal haemodynamic status are crucial for counselling parents on treatment options and possible outcomes. Clear criteria have been established for fetoscopic laser coagulation of placental vascular anastomoses, which is the treatment of choice for severe twin-twin transfusion syndrome, interrupting blood flow between the twins, and relieving uterine over-distension related to severe polyhydramnios.