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What is twin-twin transfusion syndrome (TTTS)?

Twin-twin transfusion syndrome (TTTS) is a pathologic condition that occurs in identical twins (monozygotic) sharing one placenta (monochorionic). Almost all monochorionic twins exchange blood across the placenta, although usually, the exchange is balanced. In TTTS, the vascular connections in the placenta are abnormal and the blood passes unequally between the two fetuses, with one twin (the donor twin) pumping blood to the other (the recipient twin). Without intervention, the recipient twin presents with excessive amounts of amniotic fluid (polyhydramnios), becomes overhydrated, and may develop heart failure and die; the donor twin becomes severely dehydrated, presents with decreased amounts of amniotic fluid (oligohydramnios), and may die as well. Up to 90 percent of moderate to severe cases of TTTS without intervention may result in pregnancy loss.

TTTS appears in about 15 percent of monochorionic pregnancies, amounting to 2,000 sets of twins per year in the U.S.

TTTS is categorized into five stages based on the severity of the condition.

Stage 1

In stage 1, about 40 percent of the cases will remain stable or resolve spontaneously.

  • The recipient twin presents with polyhydramnios and a measurable maximal vertical pocket of above 8 percent, AND the donor twin presents with oligohydramnios of less than 2 cm measurable maximal vertical pocket.
  • Treatment may not be necessary at this stage, but close monitoring is recommended.

Stage 2

In stage 2 and above, the disease is progressive and will probably not resolve without treatment.

  • In addition to the amniotic fluid imbalances described at stage 1, no apparent bladder is demonstrated for the donor twin. This is due to the progressive dehydration status of the donor twin, preventing it from producing any urine.

Stage 3

  • Abnormal blood flow patterns are seen in the umbilical artery or ductus venosus in either one or both twins.

Stage 4

  • The disease has progressed and accumulation of fluid known as fetal hydrops is seen as a result of cardiac failure.

Stage 5

  • In stage 5, one or both fetuses do not survive.

Early detection of TTTS and close monitoring are critical to determining the appropriate management for each case. Treatment decisions are based on the stage of TTTS, maternal cervical length, the gestational age of the twins, and their overall health. Highly specialized medical care is essential for pregnancies affected by TTTS to optimize outcomes for both twins.

At Boston Children’s Hospital, our Maternal Fetal Care Center specializes in the evaluation and treatment of TTTS and other complex fetal conditions.

Twin-Twin Transfusion Syndrome (TTTS) | Symptoms & Causes

What are the symptoms of TTTS?

In most pregnancies affected by TTTS, the pregnant person doesn’t show or feel symptoms, and the diagnosis relies solely on fetal ultrasound. However, extra amniotic fluid around the recipient twin may lead to discomfort, respiratory challenges, and premature contractions throughout the pregnancy.

What causes TTTS?

Pathophysiology is based on unbalanced vascular connections in the shared placenta, leading to net transfer of fluid from one twin to the other. In addition, the fetus’ response to altered blood volume with vasoactive mediators may worsen the condition.

Twin-Twin Transfusion Syndrome (TTTS) | Diagnosis & Treatments

How is TTTS diagnosed?

Monochorionic pregnancies require frequent and routine ultrasound evaluation to assess the health of the twins and rule out complications. Signs of TTTS may be noticed on a routine prenatal ultrasound, and other cases may be suspected when a uterus measures larger than it should for a particular week of pregnancy.

If a problem is suspected on your prenatal ultrasound, you may be referred to the Maternal Fetal Care Center for more detailed imaging.

Other tests you may encounter include:

Magnetic resonance imaging (MRI) allows further investigation of other structural anomalies and placental pathologies.

How is twin-twin transfusion syndrome treated?

There are several procedures used to treat TTTS. Which procedure or combination of procedures is right for you and your pregnancy depends on the severity of the condition.

Some cases of TTTS don’t progress past the earliest stage and thus require no intervention.

If TTTS progresses, your doctor may advise you to undergo one of the following procedures:

  • Amnioreduction/septostomy
    • amnioreduction: reduction of the excessive amniotic fluid in the recipient’s sac
    • septostomy: artificially puncturing the internal membrane to allow passive equilibrium of the amniotic fluid amounts
    • These procedures don’t treat the disease and mainly relieve some of the symptoms; they’re offered mainly in centers where more advanced treatment isn’t available or if TTTS is diagnosed later in the pregnancy (during the third trimester).
  • Fetoscopic laser photocoagulation of placental anastomoses is the treatment of choice to stop the pathophysiology. In this procedure, a fine camera (fetoscope) is introduced into the amniotic space to demonstrate the abnormal vascular connections, which are then coagulated with laser energy.

What is the long-term outlook for TTTS?

The outlook for TTTS depends on its severity and how far it has progressed. Moderate to severe TTTS pregnancies that receive fetoscopic laser photocoagulation of placental vascular connections present a significantly improved survival rate with up to 70 percent for both twins and 90 percent for at least one twin alive at birth. Survival varies depending on the disease stage, the size of the fetus, and ultrasound measurements.

With umbilical cord ligation, brain injury might be prevented in the surviving twin. Some survivors may do well and be completely healthy, while others may have injuries to their hearts and kidneys.

It is important to note that there is a risk of preterm delivery after any of these procedures.

Twin-Twin Transfusion Syndrome (TTTS) | Programs & Services