Fetofetal transfusion

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 disproportionate blood supply, resulting in high morbidity and mortality. It can affect monochorionic multiples, that is, multiple pregnancies where two or more fetuses share a chorion and hence a single placenta. Severe TTTS has a 60–100% mortality rate. 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'. Depending on the number, type and direction of the interconnecting blood vessels (anastomoses), blood can be transferred disproportionately from one twin (the 'donor') to the other (the 'recipient'), due to a state of 'flow imbalance' imparted by new blood vessel growth across the placental 'equator', the line that divides each baby's proportion of the shared placenta. This state of transfusion causes the donor twin to have decreased blood volume, retarding the donor's development and growth, and also decreased urinary output, leading to a lower than normal level of amniotic fluid (becoming oligohydramnios). The blood volume of the recipient twin is increased, which can strain the fetus's heart and eventually lead to heart failure, and also higher than normal urinary output, which can lead to excess amniotic fluid (becoming polyhydramnios).