Twin-to-twin transfusion syndrome most commonly occurs in which placentation?

• A. Monochorionic twins
• B. Dichorionic twins
• C. Trichorionic triplets
• D. Monochorionic, diamniotic twins

Answer: (A)

Twin-to-twin transfusion syndrome happens when the twins share a single placenta that contains connecting blood vessels. Those inter-twin connections allow unbalanced blood flow between the fetuses, so one becomes relatively donor with reduced amniotic fluid, while the other becomes recipient with too much fluid and volume overload. This scenario only occurs when placentation is monochorionic, meaning there is one chorion and a shared placenta. If the twins are dichorionic, each has its own separate placenta and circulations, so there isn’t a shared vascular system to cause TTTS. Within monochorionic pregnancies, TTTS is most classically described in monochorionic placentation with two amniotic sacs (and can also occur with a monoamniotic setup), but the essential factor is a single placenta with shared vessels.