Twin Anemia Polycythemia Sequence (TAPS)

TAPS is a condition that can occur in twins who share a placenta (identical, monochorionic twins). While in any monochorionic twin gestation, blood flows from one twin to the other through placental connections, in TAPS, there’s a chronic unidirectional flow. The result is that one twin (the donor twin) pumps too much blood to the other (the recipient twin). This may lead to the donor twin having too few red blood cells (anemia) and the recipient twin with too many (polycythemia). TAPS occurs in about 3 to 8 percent of monochorionic twin pregnancies, making it less common than other complications that can occur in monochorionic twins, such as twin-twin transfusion syndrome (TTTS) or selective fetal growth restriction (sFGR).

At Boston Children’s Hospital, our Fetal Care and Surgery Center specializes in the evaluation and treatment of TAPS and other complex fetal conditions.

TAPS is caused by small, one-way connections between arteries and veins in the placenta, which result in a continuous transfer of blood from the donor twin to the recipient twin. They can happen spontaneously due to unbalanced vascular connections formation or it may happen randomly or after surgery to treat twin-twin transfusion syndrome (TTTS) when some connections have been left non coagulated. Experts at the Fetal Care and Surgery Center are working to better understand exactly why these connections occur.

While a pregnant person will most likely have no symptoms indicating TAPS, ultrasound signs may indicate the disease by showing:

• A significant difference in the blood flow velocity in the twins’ brains, indicating anemia and polycythemia.
• A “starry sky” appearance in the liver of the recipient twin, indicating polycythemia.
• Difference in the echogenicity of the placenta between the donor and recipient perimeter.
   

TAPS is usually diagnosed during pregnancy using Doppler, a form of ultrasound that examines blood flow and can confirm if the blood flow is faster than usual in the donor twin’s brain and slower than usual in the recipient twin. An ultrasound can also examine how well each twin’s section of the placenta can bounce back the ultrasound signal (echogenicity), as well as look for a “starry sky” appearance of the recipient twin’s liver.

Other tests may include:

• Fetal echocardiography: An ultrasound of a baby’s heart.
• Magnetic resonance imaging (MRI): MRIs take pictures of the fetus’ brain through the pregnant person’s abdomen to determine if there is any neurological damage in either twin.

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

It’s important to closely monitor signs of TAPS and potentially intervene to prevent severe complications.

Most cases of TAPS cases can be monitored closely with ultrasound without immediate intervention. For severe cases, there are several procedures that may be considered to treat it prenatally. The management depends on the severity of the condition and parental desire. Some cases of TAPS never progress past the earliest stages, when the hemoglobin differences remain constant, Doppler measurements are normal, and there are no signs of fluid buildup in either twin’s tissues or organs.

In severe cases, our surgeons may perform one of the following procedures to improve harmful symptoms until delivery, when the twins no longer need to share a placenta:

• Endoscopic laser surgery: A procedure where through a small entry point in the pregnant person’s abdomen, a camera called an endoscope is threaded down to look at the uterus. Once the vascular connections are located, a laser is used to coagulate (clot) them, halt the abnormal transfusion, and prevent any further harm to the healthier twin.
• Intrauterine transfusion: A blood transfusion to the donor twin may help in cases that can’t be treated otherwise or when other treatments haven’t been successful.
• Selective reduction: In cases of a fetal anomaly or non-viability of a twin, there is a selective reduction of the non-viable twin in order to protect the other.
• Delivery: Doctors may deliver the babies and provide care in the Neonatal Intensive Care Unit (NICU). The timing of delivery is decided on a case-by-case basis.