Occurrence and consequences

Occurrence
Twin-to-twin transfusion syndrome occurs in 15% of identical twin pregnancies with a shared placenta, approximately one in 2,700 pregnancies. Numerous cases may go unreported, since undetected TTTS sometimes leads to miscarriage.

The most severe forms of this syndrome develop in the middle of pregnancy, between 16-26 weeks of gestation.

Consequences of untreated twin-to-twin syndrome

The consequences are different for each twin.

For the recipient:
The recipient receives too much blood. Some blood vessels of the donor divide and run across the shared placenta to the recipient, who thus receives his own blood and additional blood from the other twin. This has the following effects:

  • The heart of the recipient must work harder to pump the additional blood through the body and the placenta. This leads to enormous stress and exhaustion. The recipient may die of heart failure.
  • The increased blood volume and blood pressure cause the recipient to pass a large volume of urine, resulting in an increase of amniotic fluid. This is one of the early signs of TTTS which can be detected during an ultrasound examination. The filled urinary bladder of the recipient and the significant increase in amniotic fluid may be noted. 

For the donor:
The donor loses blood volume to the recipient via the connecting vessels and receives little blood in return. His development differs significantly from that of the recipient. 

  • The donor suffers from an undersupply of blood which may result in death if the TTTS is untreated.
  • Often the donor is called the stuck twin because there is hardly any amniotic fluid in his amniotic sac and he is pushed to the wall of the womb by the excess amniotic fluid of the other twin. This is a result of the low blood volume which causes a marked decrease in urination. An ultrasound examination will show that the urinary bladder is empty or near empty. 

If twin-to-twin transfusion syndrome is detected in the middle of pregnancy and left untreated, there is an 80-100% chance of death of the twins due to miscarriage or intrauterine death. 

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