Twin to Twin Transfusion Syndrome (TTTS)

What is TTTS?

The placenta - The developing fetus receives nutrition from the placenta through the umbilical cord. Where the umbilical cord meets the placenta it branches into many vessels that radiate out and penetrate the placenta like the roots of a tree.
Twin placentas - When there is more than one fetus is in the uterus, they must share the resources provide by the mother. Two main types of twin placentas exist, dichorionic and monochorionic.

In dichorionic twins each fetus is connected to its own separate placenta. Each placenta occupies its own separate space, like two trees growing beside each other. Although there may be competition between fetuses, there is no direct sharing of blood between twins.

In monochorionic twins, two fetuses share one placenta, like two trees growing from the same trunk. Monochorionic twins are always identical twins. Although they are usually separated by a thin wall (membrane), they have many direct connections to the placenta. These connections between the blood vessels that radiate from each cord insertion, are called vascular anastomoses (shunts). They allow the passage of blood between their circulations. These anastomoses are found in all monochorionic pregnancies - all such twins exchange blood back and forth. In uncomplicated monochorionic pregnancies there is an equal exchange of blood in both directions keeping the balance between their circulations.


Color Doppler images show blood vessel connections on the placenta.

Complicated monochorionic twins. While in most monochorionic pregnancies (remember that means two or more fetuses sharing the same placenta), sharing is equal between babies this may not always be the case. When the sharing between monochorionic twins in not equal or unbalanced it can lead to complications. Twin to twin transfusion syndrome is such a complication occurring in monochorionic twins.


Amniotic fluid volume alters imaging. The large baby on the left (recipient) has increased amniotic fluid, so a clear profile and clear imaging are obtained. The donor, on the right, is tightly wrapped in his membrane and imaging is very indistinct.

Twin to twin transfusion syndrome (TTTS) may threaten both twins in a complicated monochorionic pregnancy. In this situation, there is unequal exchange of blood, placental sharing of nutrients, fluid, and oxygen between twins. The term “twin-to-twin transfusion syndrome” (TTTS) is applied to this situation. One twin is called the donor twin and the other is the recipient twin. The donor twin sends blood and fluid to the placenta, but does not get an equal amount back from the placenta. The recipient twin also sends blood into the placenta, but gets back an excess of fluid and blood cells. This net difference leaves the donor short of fluid and placental supplies, while the recipient is overloaded.

This imbalance occurs mainly because of blood vessel connections deep in the placenta, or on its surface. An artery of one twin may connect with the artery of the other twin (arterio-arterial anastomosis). Similarly a vein of one twin can connect with a corresponding vein from the other (veno-venous anastomosis). These connections are on the surface of the placenta and permit blood exchange in both directions. Sometimes arteries coming from one twin to supply an area in the placenta (the cotyledon) do not drain into a vein returning to the same twin. Instead, the blood is drained by a vein connecting to the other twin. Such an arteriovenous anastomosis deep in the placenta allows blood flow in one direction and therefore acts like a one-way valve. If the amount of blood flow in one direction cannot be balanced by enough blood flow in the opposite direction, then an imbalance is set up. If this imbalance progresses further it can result in twin to twin transfusion syndrome.

This page was last updated on: October 3, 2007.

For detailed assessment, call the Center for Advanced Fetal Care at
1-888-349-8946 (FIX-TWIN).