Twin to Twin Transfusion Syndrome (TTTS)

TwinsTwin-to-twin transfusion syndrome (TTTS) is a rare and potentially dangerous condition that sometimes occurs in pregnancies with identical twins (or multiples) where the placenta is shared. In these type of pregnancies, TTTS occurs when blood vessels and connections develop abnormally within the shared placenta. This abnormal blood vessel formation causes uneven blood circulation between the twins. As a result, one of the twins gets too much blood flow while the other twin receives too little blood. TTTS is also called chronic inter-twin transfusion syndrome.

What is TTTS and why Does it Occur?

Twin-to-twin transfusion syndrome or "TTTS" (also known as chronic inter-twin transfusion syndrome) is a serious disorder that occurs in about 10-15% of monochorionic twin pregnancies. Monochorionic twin pregnancies occur when identical twins share a placenta. The term "chorion" is Latin for placenta and "mono" means single so monochorionic essentially means single placenta.

In a normal pregnancy, a single baby is supported by its own placenta. In fraternal twin pregnancies, each twin always has its own amniotic sac and its own separate placenta. In some cases identical twins have their own separate placentas, 70% of identical twins end up sharing a single placenta (monochorionic). When identical twins share a placenta their umbilical cords can attach to the placenta in random locations. The cords can attach right next to each other or on opposite sides of the placenta. When the umbilical cords for each twin attach too closely the blood vessels of the two cords may become connected inside the shared placenta. This abnormal connection between the cord blood vessels is ultimately what causes TTTS.

If no abnormal blood vessel connections form in the placenta, monochorionic twins will get an even distribution of blood flow from their shared placenta. However, when blood vessel from each cord abnormally connect to each other inside the placenta it can cause a dangerous imbalance in blood supply between the fetuses. The end result is that one of the twins ends up receiving too much blood circulation while the other twin is not getting enough blood. Both of these conditions can harm fetal development.

How Does TTTS Affect the Fetus?

Maternal blood supply from the placenta is absolutely critical to fetal development during pregnancy. The fetus obtains all of its oxygen and vital nutrients from the blood delivered through the placenta. When TTTS occurs one twin does not get enough blood while the other gets too much blood. This abnormal blood supply has adverse impacts on both twins.

The twin that does not receive enough blood (the "donor twin") is faced with serious dangers. When a fetus is not getting adequate blood supply, its system compensates by using blood for the most vital organs like the brain. Secondary organs, most notably the kidneys, are deprioritized and end up receiving less blood circulation. As a result, TTTS can put the donor twin a very serious risk of life-threatening kidney failure and failure of other organs due to lack of blood.

TTTS causes death of the donor twin nearly 80% of the time. The death of the donor twin can cause major problems for the surviving recipient twin because their blood vessels remain connected inside the placenta. Surviving TTTS twins are at very high risk of infant brain damage.

TTTS and Amniotic Fluid Levels

When TTTS occurs, the kidneys of the donor twin began to shut down due to inadequate blood supply. As a direct result of this kidney shut down, the donor twin produces much less urine than normal. Fetal urine is actually a key component of amniotic fluid. In fact, amniotic fluid is mostly fetal urine especially later in pregnancy. This means that when the kidneys on the donor twin shut down and produce less urine, the level of amniotic fluid surrounding the donor twin begins to drop too low. Low amniotic fluid (a condition known as oligohydramnios) can cause a number of problems and complications placing a TTTS donor twin at even more risk.

Meanwhile, TTTS causes the exact opposite problem for the recipient twin. The recipient TTTS twin is receiving too much blood from the shared placenta and responds by overproducing fetal urine. This overproduction of urine causes polyhydramnios, a condition in which the level of amniotic fluid becomes too high. Polyhydramnios can trigger a variety of obstetric complications.

Diagnosis of TTTS

Twin to twin transfusion syndrome can sometimes be difficult to diagnose, mainly because there is no way to look inside the placenta and actually see whether the umbilical cord blood vessels are abnormally connected. Instead, TTTS must be diagnosed based on clinical symptoms. The primary symptoms used to make a diagnosis of TTTS in monochorionic twins are:

  • Polyhydramnios/Oligohydramnios: differences in amniotic fluid levels between monochorionic twins is one of the primary clinical symptoms for diagnosing TTTS. Polyhydramnios in one twin and oligohydramnios in the other twin is often a key indication of TTTS.
  • Fetal Size Difference: if the twins are the same gender but one is significantly larger than the other it may be a symptom of TTTS.
  • Umbilical Cord Size: TTTS will often cause notable difference in the size of each twin's umbilical cord. The donor twin's cord will appear smaller while the recipient twin will have a larger cord on an ultrasound.
Treatment Options and Prognosis

Until fairly recently, TTTS was often a death sentence for both twins. Advancements in modern medicine have made this condition much more manageable. Doctors now have 2 options for treating and managing TTTS: amniocentesis and laser surgery.

Amniocentesis is a procedure in which doctors can tap into the amniotic sac with a small needle. This makes it possible to draw out amniotic fluid. Normally amniocentesis is done to take a very small amount of amniotic fluid for testing purposes. However, the procedure can also be used to drain or remove larger volumes of excess amniotic fluid. The use of amniocentesis to lower the levels of amniotic fluid in the recipient twin has been shown to dramatically improve blood circulation in the placenta. This greatly decreases the likelihood of TTTS resulting in premature delivery and all its related hazards. The use of amniocentesis in the treatment of TTTS has increased survival rates by nearly 60%.

The other treatment option for TTTS is fetoscopic laser surgery. This technique involves insertion of a fiber-optic scope camera through the mother's abdominal wall and into the womb itself. The camera allows doctors to actually see the abnormal blood vessel connections inside the placenta. A laser tool can then be used to basically cut out and seal off all of the abnormal connections between the twins, leaving only the normal blood vessels.