Twin-to-twin transfusion syndrome (or TTTS) is a condition that arises in 10 percent of monochorionic twin pregnancies (where twins share a single placenta) and occurs when the twins are transfusing their blood from one twin (donor) to the other (recipient). As a result one twin has very little to no fluid around it (oligohydramnios) and the other twin has too much fluid (polyhydramnios).
In some cases, TTTS can be observed with frequent checks. However, when it progresses to more advanced stages and either of the twins is showing signs of heart failure, TTTS can be fatal for both twins in more than 90 percent of cases. There are minimally invasive procedures that can be performed with little risk to the mom and markedly improved survival for the twins.