Twin-to-Twin Transfusion Syndrome

The Fetal Center at Riley Children’s Health offers laser treatment for Twin-to-Twin Transfusion Syndrome (TTTS), a rare pregnancy condition affecting identical twins or other multiples.

TTTS occurs in pregnancies where twins share one placenta and a network of blood vessels that supply oxygen and nutrients essential for development in the womb. These pregnancies are known as monochorionic pregnancies.

Sometimes the vessel connections within the placenta are not evenly dispensed and there is an imbalance in the blood exchange between the twins. One twin, the donor twin, gives away more blood than it receives in return and runs the risk of malnourishment and organ failure. The recipient twin receives too much blood and is susceptible to overwork of the heart and other cardiac complications.

The donor twin

The donor twin experiences progressive loss of blood volume (hypovolemia). Therefore, its kidneys don’t have to filter as much fluid from the blood, and as a result, urination decreases. This affects development of the bladder and leads to low levels of amniotic fluid.

Amniotic fluid, which consists of fetal urine, provides a cushion in the womb. In addition, as a fetus grows, it swallows this fluid. This aids in the development of its respiratory, urinary, and gastrointestinal systems. An abnormal reduction in amniotic fluid is known as oligohydramnios. If amniotic fluid ceases to exist this is known as anhydramnios. 

If the blood volume in the donor twin’s circulatory system is not effectively reaching the body, cardiovascular dysfunction can occur. This places the donor at risk for death.

The recipient twin

The recipient twin is at risk for successively increasing blood volume (hypervolemia). Hypervolemia leads to increased urination, more frequent bladder filling and the production of larger amounts of urine every time the bladder is emptied. This results in polyhydramnios — an abnormal increase in amniotic fluid.

A persistent state of hypervolemia ultimately affects the function of the heart muscle in the recipient, which is working hard to pump the increased amount of blood. Hypervolemia can exceed the capacity of the cardiovascular system (the heart and circulatory system of blood vessels) and can then lead to cardiovascular dysfunction and even heart failure and death.

Risk factors for developing TTTS

Being pregnant with identical twins or multiples puts a woman at risk for having a pregnancy complicated by TTTS. However, TTTS is an indiscriminate condition, occurring at random in monochorionic pregnancies.

How is TTTS Diagnosed

A maternal-fetal medicine specialist can confirm the diagnosis by conducting more detailed testing to measure amniotic fluid volume, bladder filling and blood flow in the twins.

When there is a rapid increase of amniotic fluid volume, the uterine cavity also expands at an accelerated pace, placing the mother at risk for preterm labor and shortening of the cervix. This may lead to preterm labor or preterm rupture of membranes followed by delivery. For this reason, maternal assessment of the cervical length and uterine activity is essential in all women presenting with suspected TTTS.

An important factor in determining the prognosis of TTTS is the state of cardiovascular dysfunction in the twins. This is why a diagnosis of TTTS will include a detailed examination of the fetal heart (fetal echocardiography) in both the recipient and donor.

How is TTTS Treated

In complicated cases of TTTS, minimally invasive surgery can offer the best chance of having two healthy babies. This surgery, known as Fetoscopic Selective Laser Photocoagulation (FSLP), is performed in utero between 16 and 26 weeks of pregnancy. The goal of this surgery is to correct the blood-sharing imbalance caused by TTTS.