Twin Anemia Polycythemia Sequence (TAPS)
What is twin anemia polycythemia sequence (TAPS)
Twin anemia polycythemia sequence (TAPS) is a condition in which blood flows unequally between twins that share a placenta (monochorionic), resulting in one fetus receiving more red blood cells than the other. It is similar to twin-twin transfusion syndrome (TTTS) except that the placental connections in TAPS are smaller.
Causes of TAPS
TAPS is the result of an imbalance of small connections on the surface of the placenta that allow red blood cells to be transferred from one fetus to the other. The fetus sending blood to its co-twin ends up with too few red blood cells (anemic) while the one receiving the blood has too many blood cells (polycythemic). The condition can develop spontaneously as a complication of a monochorionic pregnancy or after fetoscopic laser surgery to treat twin-twin transfusion syndrome (TTTS).
There are no known genetic causes of TAPS.
Symptoms of TAPS
In TAPS, because of the imbalance in blood flow between the twins, the recipient twin receives too many red blood cells and is at risk for developing thicker, sluggish blood, also known as polycythemia. This can lead to blood clots (thrombosis). The donor twin experiences a loss of red blood cells and is at risk for developing anemia and hydrops.
Unlike TTTS, women carrying babies with TAPS do not experience symptoms. The condition develops slowly over time, and the level of amniotic fluid remains normal throughout the pregnancy.
In some cases, the placenta on the side of the donor twin may appear thickened, and the liver of the recipient twin may develop bright white spots (known as starry sky liver).
Evaluation and diagnosis of TAPS
The most important step in evaluating a twin pregnancy for twin anemia polycythemia sequence is to determine whether the twins share a single placenta. An ultrasound examination early in pregnancy (during the first trimester) can best make this distinction.
Twin anemia polycythemia sequence can also have an impact on the cardiovascular system of either twin. It is important that twins with TAPS are thoroughly evaluated for heart problems while in utero.
If you are referred to CHOP’s Richard D. Wood Center for Fetal Diagnosis and Treatment (CFDT), you will be scheduled for a comprehensive, one-day evaluation. During this first visit, you will meet with multiple team members, including a genetic counselor, fetal therapy coordinator and maternal-fetal medicine specialist. You’ll also meet with a research coordinator to discuss ongoing research studies.
Your evaluation will involve the following exams:
- High-resolution ultrasound – An examination to rule out structural abnormalities in the twins. This exam will also confirm placental location and umbilical cord insertions into the placenta, assess amniotic fluid levels in each sac and assess blood flow patterns in specific blood vessels for each twin.
- A fetal middle cerebral arterial (MCA) Doppler study – A type of ultrasound used to measure the flow of blood through a blood vessel in the fetal brain. An MCA Doppler ultrasound can confirm whether a difference exists in the speed of blood flow between the twins.
- The donor twin will have thinner blood in the presence of anemia and the blood will move faster than normal.
- The recipient twin will have thicker blood as a result of polycythemia and the blood will move slower than normal.
- Fetal echocardiogram – A focused ultrasound of each twin’s heart to look for any related heart conditions. Twin anemia polycythemia sequence may cause serious cardiac strain in the donor twin, resulting in enlargement of the heart and cardiac dysfunction. All twins evaluated for TAPS should undergo a fetal echocardiogram.
- Genetic amniocentesis – A procedure in which a small amount of amniotic fluid is removed from the sacs surrounding the fetuses and tested. In some cases, this genetic test may be needed if imaging studies identify structural malformations in addition to TAPS.
Because TAPS develops slowly and can progress undetected, weekly MCA Doppler studies are recommended in monochorionic pregnancies, beginning between 16 and 18 weeks.
TAPS can be diagnosed after delivery by testing the red blood cell counts in the twins and analyzing the placenta for blood vessel connections.
Leiden Staging System
The Leiden Staging System is used to measure and monitor the progression and severity of TAPS. The system evaluates blood flow and cardiovascular symptoms from least severe (stage 1) to most severe (stage 5).
Your test results and specific treatment options will be discussed in detail on the day of your evaluation. If fetal intervention is recommended, you can also expect to meet with an advanced practice nurse to complete preoperative testing, and a social worker to arrange temporary accommodations in the Philadelphia area. Our team is here to help and support you and your family throughout this experience.
(Healthcare professionals: for more diagnostic tools and resources to help in the management of patients with complicated twin pregnancies, please refer to our monochorionic pregnancy resources for clinicians.)
Treatment for TAPS
Treatment for twin anemia polycythemia sequence may include the following:
- Expectant management – In situations where surgery is not yet indicated, close monitoring with periodic Doppler ultrasound examinations is used to evaluate the condition of both twins and look for signs of progression. In some cases, a follow-up fetal echocardiogram is used as well to look for signs of cardiac changes; these may sometimes be seen before other changes.
- Fetoscopic selective laser ablation – A minimally invasive surgery performed on the placenta to disconnect the communicating blood vessels. This procedure, also called selective laser photocoagulation (SLPC), stops the sharing of blood from the donor to the recipient, with the goal of halting the progression of transfusion of red blood cells.
- Intrauterine transfusion – In some instances, laser surgery is no longer possible due to late gestational age, and intrauterine transfusion for the anemic fetus may be undertaken. In these cases, diluting the blood of the polycythemic fetus must also be considered.
- Preterm delivery – When TAPS occurs later in gestation, the best option may be an early delivery to prevent further complications.
Mothers undergoing a fetal surgery procedure will stay in our Garbose Family Special Delivery Unit, the first birthing unit within a freestanding pediatric hospital dedicated to healthy mothers carrying babies with serious and life-threatening birth defects.
Follow-up care for TAPS
If you undergo fetal intervention, your care team will provide detailed postoperative care and instructions. We will schedule you for an ultrasound exam at our CFDT one week after your procedure to re-evaluate the health of your twins.
After that exam, we recommend returning to your local maternal-fetal medicine specialist for weekly ultrasound examinations for at least three weeks. Ultrasound exams will then be scheduled according to your doctor for the duration of your pregnancy.
Volumes and outcomes for TAPS
Our team at the Wood Center for Fetal Diagnosis and Treatment cares for a high volume of monochorionic twin pregnancies each year. Since 1995, more than 4,036 complicated multiple gestation pregnancies have been referred to the Center for Fetal Diagnosis and Treatment.
This experience helps to continuously expand our understanding of these complex pregnancies and our ability to differentiate between conditions such as TAPS, twin-twin transfusion syndrome, selective intrauterine growth restriction (sIUGR or sFGR), and twin reversed arterial perfusion (TRAP) sequence.
Updated February 2022
Reviewed by Juliana Sanchez Gebb, MD