Twin-to-twin transfusion syndrome occurs specifically in identical twins sharing the same placenta. It is estimated to occur in 5-10% of identical twin pregnancies. In twin-to-twin transfusion syndrome, the twins share not only the same placenta but some of the same circulation. This allows the transfusion of blood from one twin (the donor) to the other (the recipient). The donor twin becomes small and anemic, and the recipient twin becomes large and overloaded with blood.
Because the recipient twin has more blood, he/she also urinates more and has more amniotic fluid. The donor twin has less amniotic fluid; sometimes there is so little fluid that the fetus appears on ultrasound to be stuck in place on the wall of the uterus (known as “stuck twin phenomenon”).
Twin-Twin Transfusion Syndrome (TTTS) is diagnosed simply by assessing the discordance of amniotic fluid volume on either side of the dividing fetal membranes. The maximum vertical pocket (MVP) of amniotic fluid volume must be greater than or equal to 8.0 centimeters in the recipient’s sac, and less than or equal to 2.0 centimeters in the donor’s sac.
Once the diagnosis of Twin-Twin Transfusion Syndrome (TTTS) is established, the severity of the condition may be assessed using the Quintero Staging System, as listed below. This staging system is based on the observations of several hundred patients with Twin-Twin Transfusion Syndrome (TTTS). Not only does this staging system mirror the progression of disease, but it has also been shown to be important in establishing the prognosis. An atypical presentation of Twin-Twin Transfusion Syndrome (TTTS) may occur if the fetal bladder of the donor twin remains visible despite the presence of critically abnormal fetal Dopplers or hydrops.
Quintero Staging System
Stage I: The fetal bladder of the donor twin remains visible sonographically. MVP are measuring above 8CM on the recipient and below 2CM on the donor baby.
Stage II: The bladder of the donor twin is collapsed and not visible by ultrasound.
Stage III: Critically abnormal fetal Doppler studies noted. This may include absent or reversed end-diastolic velocity in the umbilical artery, absent or reverse flow in the ductus venosus, or pulsatile flow in the umbilical vein.
Stage IV: Fetal hydrops present.
Stage V: Demise of either twin.
Twin-to-twin transfusion syndrome can occur at any point during the pregnancy, even at birth (once one umbilical cord has been clamped after delivery, the other twin may get a rush of extra blood).
An additional complication known as hydrops fetalis may develop in either twin. In this condition, fluid accumulates in some part of the fetus, such as in the scalp, abdomen, lungs, or heart.
Treatment before birth
It is possible to seal off some or all of the blood vessels the twins share using an advanced laser surgery on the placenta. This balances the circulation of the fetuses and aims to end twin-to-twin transfusion. However, this requires operating while the fetuses are still in the womb, and it may cause serious complications.
There is a great video about the laser surgery at this link. It’s really fascinating.
There are other procedures which can be done, such as repeated drainage of excess amniotic fluid from the recipient twin by amniocentesis. While this procedure does not stop the cause of twin-to-twin transfusion, it can lessen the effects of the syndrome and keep the babies stable until they can safely be delivered.
If one twin is dead or dying, that part of the umbilical cord can be blocked so that blood no longer goes to that twin. The pregnancy can also be voluntarily terminated.
The most conservative treatment is to simply watch and wait. The pregnancy would be followed closely with frequent ultrasound examinations, with the option of delivering the twins by cesarean section if medically necessary.