What is Twin to Twin Transfusion Syndrome?
Twin to Twin Transfusion Syndrome (TTTS) is a disease of the placenta. It affects identical twins during pregnancy when blood passes disproportionately from one baby to the other through connecting blood vessels within their shared placenta. One baby, the recipient twin, gets too much blood overloading his or her cardiovascular system, and may die from heart failure. The other baby, the donor twin, does not get enough blood and may die from severe anemia. The babies are normal. The abnormalities are in the placenta.
Why does Twin to Twin Transfusion Syndrome Happen?
TTTS is a disease that only affects identical twins who are monochorionic-diamniotic or monochorionic-monoamniotic. See Chart Identical twins do not run in families but occur randomly. Identical twins occur when the fertilized egg (morula) splits after conception. No one knows why an embryo splits to make identical twins, so we don't know why TTTS happens. What we do know, is that the later, in days after conception, that the embryo splits, the more problems that are seen including the syndrome. The occurrence of embryo splitting is out of our control. Most twin to twin transfusion syndrome pregnancies are seen with monochorionic-diamniotic twins.
How often does Twin to Twin Transfusion Syndrome Happen?
It is difficult to answer this question with certainty, because formal tracking mechanisms to record TTTS pregnancies are only now being established. Many pregnancies go undiagnosed or unrecorded. It is crucial to have the placenta analyzed to confirm TTTS and to aid in realistic numbers. This in turn helps to further medical research and helps us prove the seriousness and urgency in fighting this condition.
From the data that is available, the rate of multiple birth is now 1 in 45 (OBSTET GYNECOL 84:101, 1994), which raises the number of babies affected by twin to twin transfusion syndrome to 6000. Looking at monochorionic identical twins, it is about 15% that will share blood vessels in such a way that the amount and direction of blood flow favors the larger recipient twin, at the expense of the donor twin. It is important for parents to understand that the babies are normal. They only become affected by what is happening in the placenta. Just as the mother has no control in when the embryo splits and how the placenta forms, the babies have no control on how they each receive their nourishment.
Many couples are waiting longer to start families and are using fertility drugs. This contributes to more twinning. (Derom C, et al: Increased monozygotic twinning rate after ovulation induction. The Lancet 1987;1:1236-8.)
At least 6000 babies are affected every year in the United States by TTTS and approximately 4000 of them will pass away.
The Recipient Twin - The Larger Baby
The recipient twin is receiving too much blood because some of the blood vessels of the other baby, in the placenta, look like a "fork in the road" and split. These vessels then cross their shared placenta over to the recipient twin. Because the recipient twin is getting his or her own blood, plus the extra blood, he becomes larger in size. This is one of the first signs of the syndrome. This extra blood is very thick like syrup. Because the blood is so thick, the baby's heart has to work harder to pump the blood. Because of the extra blood, the baby is urinating a lot. Amniotic fluid is generated by the baby urinating. This causes the recipient twin to have too much amniotic fluid. When TTTS is diagnosed midway through a pregnancy (approximately 16 to 26 weeks) 80-100% of these babies may pass away due to heart failure if nothing is done to help them.
The Donor Twin - The Smaller Baby
The donor twin is smaller and is often called the "stuck twin". This baby is getting too little blood and thus, does not urinate very much. This is why there is very little fluid in his or her amniotic sac. When born, it is often the donor twin who does better, because they are not as tired. Their heart has not had to over exert itself pumping the extra blood. However, when TTTS is diagnosed midway through pregnancy
Written by the TTTS Foundation and used with permission. StorkNet strongly encourages all families to contact the TTTS Foundation if they've received a TTTS diagnosis. This caring group provides support and information.