Etiology
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. 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"). The 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).
In early pregnancy (before 26 weeks), TTTS can cause both fetuses to die, or lead to severe disabilities. If TTTS develops after 26 weeks, the babies can usually be delivered alive and have a greater chance of survival without disability.
Other than requiring a monochorionic-diamniotic twin (or higher multiple) pregnancy, the causes of TTTS are not known. It is not known to be hereditary or genetic.
Read more about this topic: Twin-to-twin Transfusion Syndrome