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.
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.
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- 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
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.
October 3, 2007.
For detailed assessment, call the Center for Advanced Fetal