Twin-To-Twin Transfusion Syndrome
Twin-to-twin transfusion syndrome (TTTS) is a rare condition that occurs
only in identical twins during gestation. Twins normally share a
balanced exchange of blood in the womb; however, in TTTS, one twin
always donates blood to the other. In this unbalanced exchange the donor
twin is often the sicker of the two and at higher risk of dying in
utero. The donor twin may develop anemia from the blood loss to the
recipient twin, and is usually smaller and underdeveloped due to the
increased work required to pump blood for both twins. The donor twin
also produces less amniotic fluid, which can result in the twin becoming
wrapped in the amniotic membrane.
The recipient twin continuously receives extra blood and needs to cope
with this excess fluid. Typically, this twin will produce too much
amniotic fluid (polyhydramnios), and is at risk of heart failure because
of the fluid overload. Not all forms of TTTS are the same—there are
several degrees of severity and it can worsen, improve or stay the same
How common is it?
TTTS occurs in approximately 15 percent of all identical twins or 6,000
fetuses in the United States each year. The condition can also affect
triplets and higher order pregnancies; however, it only occurs in
identical siblings and never in fraternal ones.
How is it diagnosed?
TTTS is diagnosed through ultrasound and can develop at any time during
pregnancy, even as early as four months. TTTS is most dangerous in the
early stages of pregnancy when delivery is not an option.
Mothers may experience the following symptoms, all of which are due to
the excess amniotic fluid around the recipient twin:
What can happen before birth?
The recipient twin is usually larger and has too much blood, increasing
the risk for heart failure. This twin produces an excess of amniotic
fluid, which is uncomfortable for the mother and may cause premature
rupture of the membranes (“breaking the water”) or premature labor. The
excess fluid may also cause other complications in the recipient twin,
such as the retention of fluid around the lungs or heart, in the abdomen
or under the skin.
Top: normal Doppler of the umbilical artery. Middle: absent end-diastolic flow: in diastole (in between heart contractions), forward flow through the artery stops (arrows). Bottom: Reversed end-diastolic flow: in between contractions, blood flows backward (arrows).
There are a few options to treat TTTS before birth, but only the most
severe cases should be treated with fetal intervention.
Observation, with or without bed rest. If TTTS occurs in the third
trimester, conservative measures and early delivery are usually
recommended. There are typically fewer risks associated with delivering
prematurely than intervention in the womb.
What are my delivery options?
If severe TTTS develops or worsens after 26 weeks of gestation, when the
babies can survive outside the womb, it is usually preferable to deliver
early. Once they are born, the syndrome is automatically halted (since
the twins are no longer connected), but the effects of the syndrome may
not disappear right away. In most cases they are best treated after
birth in the neonatal intensive care unit (NICU).
If the syndrome improves on its own—this happens in approximately 60
percent of early stage cases of TTTS—or if laser surgery has been
performed, most physicians still prefer to deliver the babies a few
weeks before full term (at 36 to 37 weeks) to avoid further
complications. Even after endoscopic laser surgery, a Cesarean section
is not necessary other than for obstetrical reasons, which can be common
in twin pregnancies.
What is the long-term outcome?
When TTTS is severe and fetal intervention is not performed, one or both
twins die in nearly all cases. Because the twins are connected through
blood vessels in the placenta, the death of one is often followed by the
death of the other. Those twins who survive may experience severe heart
or brain damage. If laser fetal surgery is performed, at least one twin
survives in about 80 percent of pregnancies and both twins survive in
about half of those. The long-term outcome of the survivors depends on
the degree of heart failure that was present before intervention. Most
twins who survive until after birth will do as well as other infants
born at the same gestational age. However, prematurity is common with
twins, and even more so in twins who suffer from TTTS.
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