Twin-to-twin transfusion syndrome (TTTS), also known as feto-fetal transfusion syndrome (FFTS), twin oligohydramnios-polyhydramnios sequence (TOPS) and stuck twin syndrome, is a complication of monochorionic multiple pregnancies (the most common form of identical twin pregnancy) in which there is disproportionate blood supply between the fetuses. This leads to unequal levels of amniotic fluid between each fetus and usually leads to death of the undersupplied twin and, without treatment, usually death or a range of birth defects or disabilities for a surviving twin, such as underdeveloped, damaged or missing limbs, digits or organs (including the brain), especially cerebral palsy. The condition occurs when the vein–artery connections within the fetuses' shared placenta allow the blood flow between each fetus to become progressively imbalanced. It usually develops between week 16 and 25 of pregnancy, during peak placental growth. The cause of the developmental effects on a surviving fetus may include necrotic embolisms from a dead fetus, low blood volume due to pooling in the dead fetus or velamentous cord insertion (insertion of the umbilical cord into the chorioamniotic membranes). The primary treatment of TTTS is fetoscopy and laser ablation of the interconnecting blood vessels to cut off the exchange of blood between the fetuses. This treatment is associated with an 85% survival rate of at least one fetus. Other treatments include periodic removal of amniotic fluid (serial amniocentesis), which is associated with a 66% survival rate of at least one fetus. Without treatment, there is an almost 100% mortality rate of one or all fetuses. Even with treatment, the condition is associated with premature birth and a risk of cerebral palsy in a surviving fetus. Around 5–15% of identical twin fetuses will go on to develop TTTS. The condition was first described by German obstetrician Friedrich Schatz in 1875. Cause As a result of sharing a single placenta, the blood supplies of monochorionic twin fetuses can become connected, so that they share blood circulation: although each fetus uses its own portion of the placenta, the connecting blood vessels within the placenta allow blood to pass from one twin to the other. It is thought that most monochorionic placentae have these "shared connections" that cross the placenta, with the net flow volumes being equal between them. This state is sometimes referred to as "flow balance". When the placenta has deep veinartery connection, this can cause blood flow to become unbalanced. Depending on the number, type