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Twin Anemia Polycythemia Sequence (TAPS)

    • What is twin anemia polycythemia sequence?


      Twin Anemia Polycythemia Sequence  (TAPS) is a rare disease affecting monochorionic twins.

      In all monochorionic twin pregnancies, there are shared blood vessels between the babies in the placenta.   Usually, the blood flow between the babies is in balance; however, there are times where this can be unbalanced and have serious complications.

      Tiny connections in the placenta cause TAPS, less than 1mm thick, where red blood cells pass from one twin to the other.  This causes one twin, the donor, to become anemic, while their sibling becomes polycythemic, with blood like ketchup.

      Unlike twin-twin transfusion syndrome (TTTS), TAPS presents with no differences in fluids, meaning there are no physical warning signs.

      TAPS is often mistaken as a form of TTTS, but this is incorrect – they are different diseases, with different pathogenesis, diagnostic criteria, treatment options, and short and long-term outcomes.

      There are two forms of TAPS. 

      Spontaneous TAPS happens in 3-5% of monochorionic twin pregnancies.  Post-laser TAPS happens after laser surgery for Twin-Twin Transfusion Syndrome (TTTS) and is a rare complication. The Solomon laser technique has reduced the incidence of post-laser TAPS from 16% to 0-3%

      What are the symptoms of twin anemia polycythemia sequence?

      There are no physical symptoms of TAPS to a parent, unlike TTTS.  This is important to remember, as TAPS can only be detected by readings on the brain’s mid-cerebral artery and, in some later stages, by visible ultrasound markers.

      At birth, TAPS is characterized by a pale, anemic twin and a red/purple twin. 

      Before birth:

                Delta MCA-PSV doppler readings with discordance of more than 0.5 MoM

      Additional markers:

                Recipient: starry sky liver (bright spots on the liver)

                Donor: echogenic (bright appearance) to their share of the placenta

                Donor: enlarged heart (cardiomegaly)

      It is important to note that these additional markers are not present in all TAPS cases and that it is vital that routine MCA dopplers are done.  Early detection is critical.


      After birth:

                Pale, anemic donor twin

                Dark red/purple, plethoric recipient twin

                Maternal side of the placenta: donor side of the placenta is pale, recipient side is dark red and congested.

                Pathology of the placenta reveals tiny artery-vein connections less than one millimeter thick.

                Hemoglobin and reticulocyte differences


      What causes twin anemia polycythemia sequence?

      TAPS is caused by small artery-to-vein connections in the placenta, less than 1mm thick.  In spontaneous TAPS, these connections happen anywhere in the placenta.

      In post-laser TAPS, these are more likely to be a single small connection on the edge of the placenta, missed by laser surgery.

      The risks for TAPS are:

                Multiple pregnancy (TAPS has been described in dichorionic twins)

                A shared placenta.

                Previous laser surgery for twin-twin transfusion syndrome (TTTS)


      How is twin anemia polycythemia sequence diagnosed?

      TAPS is diagnosed when one baby shows signs of anemia and the other symptoms of polycythemia.  This is via Doppler ultrasound on the mid-cerebral arteries of the babies and staged using the (new) Leiden staging system as follows:

      Stage 1:  >0.5 difference in delta MCA-PSV
      Stage 2: >0.7 difference in delta MCA-PSV
      Stage 3: As per stage 1 or 2, but with cardiac compromise in the donor (abnormal doppler flows)
      Stage 4: As per stage 1 or 2, but with hydrops in the donor
      Stage 5:  Death of one or both twins

      In the above description, “Delta” 0.5 and 0.7 refer to MoM readings with a difference greater than 0.5 or 0.7. For instance, MoM readings of 0.9 in one twin and 1.6 in the other twin could indicate Stage 2 TAPS because those readings are different by 0.7.

      MCA dopplers test the speed blood moves through the brain and are a reliable indicator of polycythemia and anemia.

      After birth, TAPS is diagnosed using a different staging system, reliant on blood testing. 

      To accurately diagnose TAPS, the babies’ hemoglobin and reticulocytes (promille) must be checked.  The recipients’ reticulocyte count should be divided by the donor’s, and if that number is greater than 1.7 and:

      Stage 1: Hemoglobin difference of more than 8g/dL
      Stage 2: Hemoglobin difference of more than 11g/dL
      Stage 3: Hemoglobin difference of more than 14g/dL
      Stage 4: Hemoglobin difference of more than 17g/dL
      Stage 5: Hemoglobin difference of more than 20g/dL

      The placenta should be flipped over to check for a color difference and sent to pathology for dye injection and analysis to look for tiny connections.

      What are the treatments for twin anemia polycythemia sequence?

      There is no best treatment for TAPS at this stage, but treatment options exist, and all should be offered and discussed with parents to make an informed choice. 

      Laser Surgery

      Laser surgery with the Solomon technique is widely used and accepted as an effective treatment for TAPS.  Solomon laser draws a line down the equator of the placenta, effectively cutting off any small connections.


      Used later in pregnancy, transfusions, and exchanges are used to buy some extra time.  The donor is given a blood transfusion in utero, and the recipient’s blood is thinned out.  This does not fix the cause of the problem, though, and only buys a little extra time.

      Expectant Management

      Because of the slow-moving nature of TAPS, watch and wait is a commonly used approach.  This is mainly used in the later stages of pregnancy.

      Premature Delivery

      If the babies have reached viability, they can be delivered early to help with treatments.

      Selective Reduction

      Unfortunately, in some advanced cases of TAPS, the option to reduce or terminate the pregnancy can be made.  This is when there are visible signs of life-threatening injuries or imminent fetal demise.

      Ultimately, it comes down to the mother and the babies’ health and the stage of pregnancy.

      After birth

      After the babies are born, it is crucial to complete the proper diagnostic process to establish that it is TAPS (as acute TTTS has a different treatment protocol, and the wrong treatment can be dangerous or even fatal)

      Blood transfusions for the donor and partial exchange transfusions for the recipient are the usual treatments and correct any blood imbalances.

      Short and Long Term Effects of Twin Anemia Polycythemia Sequence

      For post-laser TAPS, the long-term effects are similar to TTTS. One in five twins who survive post-laser TAPS will have some degree of neurodevelopmental impairment or cognitive delay.  There is no difference between donors and recipients.

      Post-laser TAPS has a high perinatal mortality rate, with 26% of donor twins passing before or shortly after birth.


      Spontaneous TAPS has different long-term outcomes, with donor twins four times more likely to have some degree of neurodevelopmental impairment or cognitive delay.  44% of TAPS donors have some degree of neurodevelopmental impairment. 1 in 8 spontaneous TAPS donors will also be born deaf, a phenomenon not reported in other cases of fetal anemia, such as TTTS or rhesus disease.


      Where can I find out more about twin anemia polycythemia sequence?


      NORD Twin Anemia Polycythemia Sequence

      The TAPS Trial

      The TAPS Support Foundation

      Fetal Medicine Foundation

      Fetal Health Foundation

TAPS Articles