A complication of monochorionic multiple pregnancies in which one fetus receives more blood flow than the other from the placenta. It is diagnosed by fetal ultrasonography: one twin’s amniotic sac has polyhydramnios (excessive amniotic fluid), while the other twin’s sac has oligohydramnios (insufficient amniotic fluid). Death of one or both twins will occur without intervention. Treatments include repeated amniocenteses, laser therapy to prevent the exchange of blood between twins, or intrauterine surgery.