TAPS in Twins: Twin Anemia Polycythemia Sequence Explained

| Reviewed By Sarah Schulze, MSN, APRN, CPNP

Since TAPS was first discovered in 2006, it has been a source of worry for numerous parents who are pregnant with identical twins.

What is TAPS in twins? TAPS is a rare condition that occurs in twin pregnancies and involves an unequal blood count between the two developing fetuses in the womb. Basically, one twin essentially takes red blood cells from the other twin, causing an imbalance in the concentration of both red blood cells and hemoglobin. 

So what exactly happens if twins have TAPS? What can you expect? Read on to learn a little bit more about this prenatal syndrome. 

Twin Anemia Polycythemia Sequence (TAPS Syndrome)

TAPS is unique to monochorionic twin pregnancies, where twins share one placenta and a network of blood vessels. 

The placenta is the lifeline of the unborn babies. It is made of many blood vessels, arteries, and veins that facilitate the exchange of oxygen, blood, and nutrients from the mother to the growing babies through separate umbilical cords.

TAPS is caused when a few of these small connections join together, messing up the equal distribution of blood to the growing fetuses. 

The twin that loses blood is called the donor twin while the receiving twin is called the recipient twin.

Because the recipient twin is receiving so much “extra” blood, he or she will have a slower-than-normal blood flow and is at risk for polycythemia, which is a successively increasing red blood cell count.

The donor twin will have a faster blood flow and is at risk for anemia, which is a lower-than-normal red blood cell count.

Red blood cells play a primary role in delivering oxygen to the developing tissues and organs of the fetuses, hence the name, Twin Anemia Polycythemia Syndrome. 

TTTS in Twins

Twin-to-Twin Transfusion Syndrome (TTTS) is another rare prenatal condition that affects monochorionic twin pregnancies.

It is similar to TAPS in the sense that it occurs when there is an uneven distribution of blood and oxygen between the twins. 

When there is an imbalance in the blood exchange between twins, the donor twin gives away more blood than it receives and runs the risk of malnourishment, which can lead to organ failure.

On the other hand, the recipient twin receives too much blood and is susceptible to cardiac complications from the cardiovascular system overworking to pump all of the extra blood. 

Exceeding the cardiovascular capacity too much can lead to heart failure and the death of the recipient twin. 

An additional complication with TTTS is the amniotic fluid levels, which are greatly affected by blood count.

When the donor twin experiences progressive blood loss, the kidneys do not have to filter as much fluid from the blood, and therefore, urination decreases. This leads to low levels of amniotic fluid that can be detrimental for the growing twin.

On the contrary, the recipient twin is receiving a ton of blood that leads to increased urination and an abnormal increase in amniotic fluid. 

What’s the Difference Between TAPS and TTTS?

These two prenatal conditions are so similar that it can be very difficult to decipher between the two. 

The biggest difference between them is that TTTS has other symptoms and is often characterized by an imbalance in the amniotic fluid between the babies, while fluid levels remain unchanged with TAPS. 

You can think about TAPS as more of a “slow leak” of blood cells from one baby to the other while TTTS is usually more drastic. There are no physical symptoms of TAPS in the babies as there are with TTTS. 

What Causes TAPS in Twins? 

Within the placenta, there are tiny blood circulations (anastomoses) through which blood, oxygen, and other nutrients move and flow in order to support the growing babies.

Ideally, these circulations facilitate an even exchange from one baby to the other, and the twins develop equally.

However, sometimes the tiny placental blood circulations join together and cause an imbalance in red blood cells that are exchanged between the twins. 

Can You Prevent TAPS in Twins?

TAPS is not caused by anything that the mother or doctors do during pregnancy. It is a rare condition that occurs because of abnormal circulatory connections in the placenta that are beyond anyone’s control.

The best thing to do if you are at risk of developing TAPS is to receive regular prenatal care where they will monitor your twins’ blood flow frequently using Doppler ultrasound to detect any discrepancies and catch TAPS early. 

How Common Is TAPS in Twins? 

TAPS occurs spontaneously in 3-5% of monochorionic twin pregnancies. Post-laser TAPS can develop due to residual anastomoses in 2-16% of pregnancies treated with laser surgery for TTTS. 

TAPS in Twins Symptoms

It is not uncommon for TAPS to go undiagnosed before birth due to the lack of symptoms and unaffected amniotic fluid levels like in TTTS.

The only real way to detect TAPS is through Doppler ultrasound focusing specifically on the blood flow in the middle cerebral artery (MCA).

An increase in the speed of blood flow in the MCA of one twin can be a sign of fetal anemia, making this twin the donor twin.

A decrease in the speed of blood flow in the MCA of the other twin is a sign of polycythemia.

There are other markers that can be seen on ultrasound such as a “starry sky” liver in the recipient or the donor twin’s half of the placenta looking brighter. However, the most reliable determinant of TAPS is routine MCA dopplers. 

At birth, the signs and symptoms are more evident. The donor twin is usually strikingly pale from loss of blood while the recipient is usually dark red/purple in color from an excess of blood. 

How Is TAPS Diagnosed?

In most cases, TAPS is diagnosed during pregnancy using Doppler, which is a form of ultrasound used to examine blood flow. This examination can confirm if the speed of blood flow is faster for one twin and slower than normal for the other twin.

Many TAPS cases go undiagnosed until after birth. Once doctors see an extreme color difference in the babies, they may begin tests for TAPS by examining the placenta.

The placenta can be injected with colored dye to detect the tiny connecting blood vessels. The maternal side of the placenta will show an extreme color difference with a pale side of the donor and a darker side of the recipient. 

TAPS Twins Stages

Since its discovery in 2006, a lot of research has been done on this rare prenatal syndrome, and researchers have begun to understand it more. In 2019, a revised staging system was proposed and is used today to classify the levels of TAPS.

The system proposes 5 levels to classify TAPS cases with stage 5 being the most severe.

Stages 1 and 2 are classified by the difference in the readings between the twins’ blood counts. The other three stages are classified by progressive complications.

Can Twins Survive Taps?

There are many twins who do survive TAPS and lead normal, healthy lives. However, there are also many twins who survive but suffer adverse effects and complications related to prematurity or face long-term effects.

TAPS Prognosis

TAPS can be highly unpredictable, and the outcomes cannot always be given directly. Each case is very different and can have different adverse effects. 

However, there are some things you can keep in mind or possibly expect if your twins are diagnosed with TAPS. 

  • Routine doppler ultrasounds to monitor blood flow and cardiac activity
  • Pre or postnatal blood transfusions 
  • Premature birth
  • Possible complications including a high chance of neurodevelopmental impairment and deafness in donor twins
  • Potential death and injury to babies

TAPS Survival Rate

Current research predicts a survival rate of 83% for twins diagnosed with TAPS.

TAPS Treatment

While TTTS is often treated with laser surgery, there is not one optimal treatment for TAPS because it is still such a “new” condition.

There are still a few things that can be done to preserve the lives of the growing babies. Regarding treatment, patients should talk with their doctor and be guided by the expertise of their teams. 

  • Expectant management – This is basically a watch-and-wait approach. You will be monitored closely every few days, but there will be no intervention unless needed. 
  • Laser surgery to sever connections
  • Intrauterine blood transfusions 
  • Early delivery 
  • Selective reduction/termination

Long-Term Effects of TAPS

Research is being done on the long-term effects of TAPS and what can be expected. The most common long-term effects being studied include deafness and neurodevelopmental impairment in donor twins. 

Currently, there is a 15% chance that donor twins will have deafness caused by Auditory Neuropathy Spectrum Disorder and a 31% chance of neurodevelopmental impairment. 


There is still much to learn about TAPS, and researchers are working hard to give us more information about this rare but frightening condition.

As scientific technology continues to improve and more information is found, it is our hope that more TAPS patients can have the best chance of receiving treatment and living normal, healthy lives.