Twin-to-Twin Transfusion Syndrome: Definition & Diagnosis

Instructor: Courtney Dohse
In this lesson, we will learn the definition of Twin-to-twin transfusion syndrome (TTTS) as well as the method used to diagnosis it. We will also briefly review complications of this syndrome for both babies.

Case Study

Beth has just learned at an early ultrasound that she is pregnant with identical twins. The sonographer informs Beth that her twins share a placenta. The obstetrician has Beth come back for a follow-up ultrasound a few weeks later, which reveals that Twin B is measuring significantly larger than Twin A. Beth's doctor gives her some educational materials on Twin-to-twin transfusion syndrome and notifies Beth that this pregnancy is considered high risk. Beth is then scheduled for weekly follow-up ultrasounds and non-stress tests for the rest of her pregnancy.


Twin-to-twin transfusion syndrome (TTTS) is a complication of pregnancy in which identical twins, who share a placenta, have intertwined or connected blood vessels resulting in the shunting of blood from one twin to the other. This syndrome occurs in approximately 15% of women who are pregnant with identical twins. TTTS is problematic because it causes an unequal sharing of blood and nutrients from one twin to the other. Looking back at our case study, since Twin B is measuring much larger, he would be considered the recipient twin, because he is receiving both his own blood supply from the placenta, as well as the blood that the other twin is pumping to him. Twin A in this case, who is measuring much smaller, would be considered the donor twin since he is pumping his share of the blood to his twin.

Twin-to-twin transfusion syndrome

This syndrome can cause complications for both of the involved babies. In this example, complications for Twin A (the donor twin) include oligohydramnios, or low amounts of amniotic fluid. He may also have a small or absent bladder. For Twin B (the recipient twin), since he is receiving an oversupply of blood, he may have an overabundance of amniotic fluid, or polyhydramnios. Due to the extremely high blood volume, this baby may also develop an enlarged bladder or heart failure due to the inability to keep up with the oversupply of blood.


TTTS is diagnosed through an early ultrasound, as seen in our clinical case study. In order for a diagnosis of TTTS, the twins must be identical, be monochorionic or share a placenta, and one twin must be at least 20% larger than the other twin.

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