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Twin-Twin Transfusion Syndrome (TTTS)

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What is twin-twin transfusion syndrome (TTTS)?

Twin-twin transfusion syndrome (TTTS), also called twin-to-twin transfusion syndrome, is a rare disorder that affects identical twin pregnancies. It occurs when the two babies share a placenta in the womb. This is also referred to as a monochorionic twin pregnancy. It most commonly occurs in monochorionic twin pregnancies when the babies have two separate amniotic sacs.  This is referred to as monochorionic-diamniotic (one placenta, two amniotic sacs).

In TTTS, blood vessels from the twins' umbilical cords form connections within their shared space, linking one baby to the other. When this happens, blood flows unevenly between the babies and their circulation can become unbalanced. This puts the twins at risk for early delivery, neurological damage, heart damage and can even be fatal.

In TTTS, one baby is referred to as the "donor" twin. This twin "donates" their circulation of fluid and blood (referred to as blood volume) to the other baby, who we call the "recipient" twin. As a result, the donor twin may stop producing urine and its amniotic sac becomes much smaller.

The recipient twin receives too much blood volume and therefore produces more urine leading to more amniotic fluid, resulting in having a very large amniotic sac.

TTTS can lead to severe complications for the twins. That's why early detection is critical to help improve outcomes for both babies and their mother.

What causes twin-twin transfusion syndrome during pregnancy?

The exact cause of TTTS is not well understood; there are no known genetic causes of TTTS, and there's nothing the mother did to cause it. In most cases, TTTS develops in the second trimester of pregnancy.

While there's nothing anyone can do to prevent TTTS, there's plenty we can do to help your babies. The important thing families can do when they receive the diagnosis of monochorionic-diamniotic twins, meaning the babies share a placenta but have two separate amniotic sacs, is to closely watch for TTTS. Families should have fetal ultrasounds evaluating for signs of TTTS every two weeks starting at 16 weeks gestation until the end of pregnancy.

How common is TTTS?

TTTS affects identical twins before they are born. Approximately 1 in every 250 pregnancies are identical or monochorionic and approximately 2 out of 3 of these pregnancies are monochorionic-diamniotic. Of these pregnancies, 10% to 15% develop TTTS.

Although less common, TTTS can happen in monochorionic-monoamniotic twins where the twins are in one sac without a dividing membrane.

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