What is Twin-to-Twin Transfusion Syndrome?
Twin-to-Twin Transfusion Syndrome (TTTS) is a condition that occurs in identical twins that share a common placenta (monochorionic twins). Connections between the twins on the surface of the placenta are common, and are usually in balance, with blood flowing in both directions equally. On occasion, the connections are unbalanced, and one baby is chronically losing or donating blood to his or her sibling. The baby losing the blood is called the “donor” and the one receiving the blood is called the “recipient”.
Left untreated, overtime the donor is receiving insufficient nutrition from the placenta, leaving it smaller and dehydrated. The amniotic sac (comprised of fetal urine) becomes empty and membranes wrap closely around the baby. When this happens, typically the donor is described as “stuck.”
On the other hand, the “recipient” is receiving too much nutrition and blood from the placenta and his or her amniotic sac becomes full with excessive amounts of amniotic fluid. This extra amniotic fluid often causes the uterus to become distended rapidly. Eventually, the recipient twin can develop heart failure when he or she can no longer cope with the extra blood volume.
Without treatment, neither twin survives in 80 to 90% of the cases. The outcome is dramatically improved with treatment.
How common is it?
Twins occur naturally in 1 in 84 pregnancies. About one third of twins are identical and two thirds of them share the same placenta. 10-20% of these twins end up with some degree of unequal sharing of blood. That means one in every 2, 000 pregnancies may have one Twin-to-Twin Transfusion Syndrome pregnancy.
How is it diagnosed?
TTTS is usually diagnosed by ultrasound early in the pregnancy. The characteristic appearance of low fluid in one sac and excess fluid in the other must always be present for the diagnosis of TTTS. The fluid levels are measured by looking for the biggest pocket of fluid around each baby in the amniotic sac, which is called the maximum vertical pocket, or MVP. The MVP for the donor twin must be less than 2 cm, and the MVP for the recipient twin must be greater than 8 cm. As the condition advances, the bladder of the donor becomes too small to see, and the blood flow in the umbilical cord and heart of the babies becomes abnormal. In advanced cases, the recipient appears swollen (a condition called hydrops) and one or both of the twins may spontaneously die.
What treatments are available?
There are a number of treatment options for TTTS. Due to the potential for poor outcomes, some families may opt to terminate the entire pregnancy, if TTTS is diagnosed early. Alternate therapies include serial amnioreductions, which attempt to equalize the pressure in each sac and helps to reduce the risks of preterm labor and fetoscopic laser photocoagulation, an in-utero surgical procedures that seals the connections between the babies.
If you would like to discuss a particular fetal condition or arrange a consultation, please call 425.899.3537 or email the program coordinator at: mldorn@evergreenhealthcare.org.