Twin Anemia Polycythemia Sequence (TAPS)


What is Twin Anemia Polycythemia Sequence (TAPS)?

Twin anemia polycythemia sequence (TAPS) is a rare complication that occurs in pregnancies with identical twins who share a placenta.

In TAPS, there is an imbalance in red blood cells between the twins. One twin has too few red blood cells (anemia), while the other twin has too many red blood cells, leading to thick blood (polycythemia).

A Form of Twin-Twin Transfusion Syndrome (TTTS)

TAPS is a form of twin-twin transfusion syndrome (TTTS), a serious condition that can result in uneven blood flow between identical twin babies.

Both TAPS and TTTS occur in pregnancies where identical twins share a single placenta, known as a monochorionic twin pregnancy. In both conditions, abnormal blood vessel connections within the shared placenta allow one twin to transfer or pump blood to the other.

In TAPS, this inter-twin transfusion occurs slowly through a few miniscule blood vessel connections. Blood flows from the “donor” twin, depriving that baby of red blood cells, to the “recipient” twin, who receives an increasing amount of red blood cells.  

Cause and Risk Factors

While the cause of TAPS is unknown, the complication can occur:

  • Spontaneously in pregnancies involving identical twins who share a single placenta
  • Following fetoscopic laser surgery for the treatment of twin-twin transfusion syndrome (TTTS)


Untreated, the progression of TAPS can place both twins at risk of serious complications and threaten the survival of one or both babies. The donor twin may not receive the oxygen and nutrients required for fetal development, while the recipient twin’s blood may become too thick, leading to a risk of blockage or excessive strain on the heart.


Signs and symptoms of TAPS may include:

  • A low red blood count (anemia) in the donor twin and thicker than normal blood (polycythemia) in the recipient twin
  • Slow blood flow in the recipient twin
  • A blood clot (thrombosis) in the recipient twin

Unlike twin-twin transfusion syndrome (TTTS), in TAPS the amniotic fluid levels are normal.

Testing and Diagnosis

In most cases, TAPS is diagnosed during pregnancy using Doppler, a form of ultrasound used to examine blood flow. The test can confirm if the speed of blood flow is faster than normal in one twin (the donor) and slower than normal in the other (the recipient). A fetal echocardiogram may also be performed. 

A 5-level staging system, known as the Leiden staging system, is used to classify the severity of TAPS during pregnancy, with stage five being most severe.

If TAPS is undetected during pregnancy, the condition may be diagnosed after delivery based on an imbalance of red blood cells in the twins and inspection of the placenta for abnormal connections between blood vessels.


Appropriate treatment will depend on the health of the mother and the condition of the fetuses.

In-utero treatment options include:

  • Close monitoring and management, in cases where immediate intervention is not required or laser surgery is not an option
  • Blood transfusion to the anemic donor twin, a temporary treatment that can possibly worsen the condition of the recipient twin with polycythemia
  • Fetal laser surgery to close off and separate the connected blood vessels, stopping the transfer of blood from one twin to the other

Laser surgery is the only treatment option that can stop the cause of the red blood cell imbalance. In this minimally invasive procedure, a small camera known as a fetoscope is used to inspect the placenta, locate the abnormal blood vessel connections, and disconnect them using laser energy. The procedure, known as fetoscopic laser ablation, is highly challenging because the blood vessel connections are extremely small, making detection difficult.  

For babies diagnosed with TAPS at birth, treatment involves addressing any blood imbalance complications, including a possible blood transfusion for the donor twin with anemia.

Texas Children’s Fetal Center - Your First Visit

Texas Children’s Fetal Center provides diagnostic imaging and treatment recommendations on the same day. Depending on the severity of TAPS, fetal intervention can be scheduled immediately when indicated.

Initially, the patient will undergo an ultrasound, and if necessary, an MRI and fetal echocardiogram to confirm the initial diagnosis and stage of TAPS. Then, the patient and family will meet with their care team to discuss a comprehensive plan of care. The team consists of fetal therapy and surgery physicians, a nurse coordinator and a fetal cardiologist. During this meeting, the individualized treatment plan for each patient is discussed and options are presented, including whether surgical fetal intervention is necessary.

Volumes and Outcomes

In the News

Twins Received Lifesaving Procedure at Texas Children's Before They Were Born – ABC 13 KTRK

For more information or to schedule an appointment,

call Texas Children’s Fetal Center at 832-822-2229 or 1-877-FetalRx (338-2579) toll-free.

Our phones are answered 24/7. Immediate appointments are often available.