What is Twin-to-Twin Transfusion Syndrome?

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Twin-to-twin transfusion syndrome (TTTS) is a specific complication unique to multiple pregnancies that have just a single placenta (monochorionic twins). It develops when there is an unbalanced transfer of blood from one foetus to the other by means of abnormal, interconnecting blood vessels (anastomoses) running through the shared placenta. The foetus transferring the blood is known as the “donor”, while the foetus accepting it is called the “recipient”.

Is twin-to-twin transfusion syndrome very common?

There is one monochorionic (MC) twin pregnancy in every 250 pregnancies, they correspond to around 20% of all twin pregnancies and 70% of monozygotic pregnancies (those derived from a single ovum).

The incidence of twin-to-twin transfusion syndrome is 10–15% of monochorionic pregnancies and so it affects approximately 1 in every 3,000 pregnancies.

Types of twin-to-twin transfusion syndrome

The classification is based on ultrasound data and categorises the severity of the syndrome. It is divided into 5 stages (I–V). In stages I and II the foetuses do not present any severe cardiovascular alterations. More severe alterations start to develop in stage III and by stage IV one of the foetuses will present advanced heart failure.

  • Stage I. There is amniotic fluid imbalance between the donor and recipient (polyhydramnios/oligohydramnios). The bladders also present different sizes; the recipient has a full and swollen bladder throughout the ultrasound examination, whereas the donor has a smaller bladder.
  • Stage II. The donor foetus’ bladder is no longer visible.
  • Stage III. This is a more advanced stage of the disease in which alterations appear in the recipient and/or donor foetus’ circulation; these are the result of haemodynamic changes secondary to an increase (hypervolaemia) or decrease (hypovolaemia) in the total volume of blood circulating in each foetus. The most common alterations are:
    • Absent or reversed end-diastolic flow in the donor and/or recipient’s umbilical artery.
    • Absent or negative A wave in the ductus venosus (usually in the recipient).
    • Umbilical vein with pulsatile flow (usually in the recipient).
  • Stage IV. Hydrops in either of the foetuses (accumulations of fluid in two or more foetal compartments, e.g., subcutaneous, pleural effusion, ascites, pericardial effusion).
  • Stage IV. Intrauterine death of one or both twins.
Substantiated information by:
Mar BennasarDoctor specialist in Fetal Medicine — Maternal–fetal Medicine DepartmentMaría MaríMaría Marí — Maternal–fetal Medicine Department

Published: 20 February 2018
Updated: 20 February 2018

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