Brown University

Fetal Medicine @ Brown

The Warren Alpert Medical School of Brown University

Twin to Twin Transfusion Syndrome
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The text below will, hopefully, answer some of the questions you may have regarding this condition. You can scroll down, or go directly to a specific question:

Underlined terms are explained in detail - just click on them. A medical review on this topic was recently published by Drs. Carr and Plevyak in Medicine & Health/Rhode Island.

 For further questions, you can contact the Fetal Treatment Program Coordinator at (401) 228-0559 (beeper (401) 452-4469) or at


Twin-to-twin transfusion occurs in a monochorionic pregnancy (i.e., identical twins who share a single placenta) when blood from one fetus circulates to the other twin. In general, identical twins normally exchange some blood during gestation; this exchange is usually balanced, so that one twin will act as the 'blood donor' one moment, and as the 'recipient' the next. The twin-to-twin transfusion syndrome occurs when one twin always 'donates' blood to the other, because the communication between the two is unbalanced.

Twin-to-twin transfusion syndrome does not occur in non-identical twin fetuses, where each fetus has its own placenta. It is also much less likely to occur if both twins are not only identical, but share a common amniotic space (so-called mono-amniotic twins). The latter is the least common form of twin pregnancies, but is associated with other potentially life-threatening problems.

In the twin-to-twin transfusion syndrome, one twin (donor) will have to pump blood, not only for himself, but also to transfuse the recipient twin. The donor has to do more work, thereby having less energy to grow; he will show signs of intrauterine growth retardation, will produce less amniotic fluid (oligohydramnios), and eventually become almost wrapped in its amniotic membranes: there is now so little fluid surrounding him that he is 'stuck.' Hence, twin-to-twin transfusion syndrome is sometimes referred to as 'stuck twin' syndrome.

The recipient twin, on the other hand, ends up with too much blood and fluids, which forces him to eliminate it through the urine: a recipient twin will therefore often be seen with a full bladder. The excess fluid causes polyhydramnios, the excess accumulation of amniotic fluid in this fetus's amniotic space. The fetus himself will tend to swell (become edematous), and may go into heart failure, or 'hydrops'. The combination of polyhydramnios in the recipient and oligohydramnios in the donor is also referred to as 'poly-oli' syndrome, another term for twin-to-twin transfusion syndrome.

Figure 1 illustrates a normal situation, where blood flows in either direction. Figure 2A illustrates the onset of twin-to-twin transfusion syndrome, where blood preferentially flows from one fetus (the donor) to the other (the recipient). In figure 2B, the full-blown syndrome ("stuck twin") develops.

Figure 1: Twins A and B each have their own amniotic cavity, but a single placenta. Blood can flow from twin A to twin B, or vice-versa: there is balanced transfusion.

Figure 2A: Twin A (donor) is transfusing twin B (recipient), without counterbalanced transfusion from B to A.


Figure 2B: Twin A remains smaller, develops less amniotic fluid (becomes the 'stuck' twin). Twin B develops hydrops (swelling) and large amounts of amniotic fluid (polyhydramnios).



It happens in 5 to 15% of all identical twins, and puts about 6,000 babies at risk in the United States each year.* It can happen anytime during the pregnancy. It can also affect triplet and higher order pregnancies.


If twin-to-twin transfusion syndrome occurs late in gestation, the risks are usually minimal; if one fetus is threatening to develop hydrops or failure, delivery of the babies might be the best option. If this occurs earlier, before the babies are mature enough to survive out of the womb, the options are fewer and the risks greater.

Preterm, premature rupture of the membranes (PPROM) and premature labor are often a threat to the twin pregnancy, possibly due to increased pressure and increased amounts of amniotic fluid (polyhydramnios in the 'recipient' fetus). If the babies are born prematurely, they may be at a higher risk of suffering serious complications, or even dying in infancy.

One or the other twin is at a high risk of suffering severe heart- or brain problems (due to insufficient or inappropriate blood flow caused by the transfusion). The donor twin is usually the sickest; he will often be anemic (not enough red blood cells), and is the one at highest risk of dying in utero.

The recipient twin may suffer as well, and develop hydrops (heart failure) due to fluid and blood overload. In addition, chronic transfusion may make the recipient's blood thicker (more viscous). This, in combination with somewhat sluggish blood flow because of heart failure, may place the recipient at risk for "vascular accidents" whereby a blood vessel becomes blocked. This may theoretically affect any organ, or an entire limb, causing necrosis. As a result, the organ may be damaged or even absent. Examples of this are an absent kidney, an interruption in the intestinal tract or a severely abnormal leg.

Once a fetus dies, the remaining twin is at a very high risk of dying within days. If he survives, he is very likely to develop very severe heart or brain damage.

If twin-to-twin transfusion syndrome develops early in pregnancy (before 20 weeks) and nothing is done, there is a very high likelihood that both twins will die.


No, some cases of TTTS are extremely severe and rapid in onset, while others have a less aggressive course. There are several ways doctors can describe and classify the severity of the syndrome, but the most commonly used is the one devised by Dr. Ruben Quintero. The Quintero classification (or its modifications) uses ultrasound findings to estimate the severity of TTTS:

In stage I, there is severe polyhydramnios around the recipient and severe oligohydramnios around the donor, but the donor is still able to produce enough urine to fill his bladder (which is visible on ultrasound).

In stage II, the degree of transfusion and dehydration of the donor is such, that he is no longer able to fill his bladder - the bladder is now no longer visible on ultrasound.

In stage III, additional signs of fetal stress are seen: there may be pulsatile (rather than uniform) flow through the umbilical vein (usually of the recipient), absent or even reversed end-diastolic flow in the umbilical artery (usually of the donor) or other signs of cardiovascular stress, such as a leaky heart valve (tricuspid regurgitation, or TR). As a group, these signs are called "critical dopplers," because they are usually diagnosed by using Doppler ultrasound (a feature of the ultrasound machine).

In stage IV, there are now overt signs of heart failure in one or both twins: fluid starts to accumulate around the heart (pericardial effusion), around the lungs (pleural effusions), in the abdomen (ascites) or under the skin (edema, or thickened nuchal fold).

Clearly, stages III and IV are more severe than stages I and II. In general, surgical intervention (such as laser) is not recommended in lesser stages, and most centers will only offer aggressive treatment for stages II, III or IV. The main reason for this is that the prognosis is better for stage I than for higher stages (provided that the twins remain in stage I and do not progress to a higher stage).

Unfortunately, it is very difficult, at this time, to predict how a particular case will progress: some twins have been seen to remain in stage I for many weeks, or even improve on their own; while others have progressed very rapidly to a higher stage, sometimes even skipping one or more stage. Much of the current research in TTTS is now directed at understanding why some twins develop the syndrome and others don't, and why some progress more rapidly than others.


1. Observation and bedrest.

If twin-to-twin transfusion syndrome occurs after 25 to 28 weeks, conservative measures (bedrest, single amnioreduction) and early delivery are usually recommended. Amnioreduction means the removal, through a fine needle, of the excess amniotic fluid from the recipient twin with polyhydramnios (see below). As the pregnancy comes closer to term, it may be best to 'wait and see,' and to plan early delivery if either twin shows signs of distress. In this situation, the risks associated with (mild to moderate) prematurity may be smaller than the risk of an intervention in the womb (such as the ones described below).

2. Amnioreduction.

Removal of excess amniotic fluid from the recipient twin has been, until recently, the best available treatment for twin-to-twin transfusion syndrome. The reason for its effectiveness is not completely clear, but removing the excess amniotic fluid may help decrease the risk of rupture of the membranes (PPROM) and premature labor. It may also be beneficial by relieving the pressure on the umbilical cord of the twin with excess fluid, and thus improve blood flow between the fetus and the placenta.

Amnioreduction does not, however, treat the cause of twin-to-twin transfusion syndrome, only one of its effects. Since the fluid is likely to accumulate again (usually within a few days to a week or two), the procedure will have to be repeated. With each amnioreduction, the risk of bleeding or infection increases, as does the risk of injury to the membranes.

The results of repeated amnioreduction depend on how often this has to be done, and how rapidly fluid accumulates again. In general, the earlier in gestation the syndrome develops and the quicker polyhydramnios recurs after amnioreduction, the worse the outcome. Whereas survival of either twin in twin-to-twin transfusion syndrome is very low if the condition develops early (before 20 weeks of gestation), repeated amnioreduction can improve survival of at least one twin to 50-60%. Unfortunately, the risks of severe complications in the surviving twins may be as high as 20 to 35%, and includes severe heart or brain anomalies.

3. Laser coagulation of the communicating placental vessels.

This technique, a form of fetal surgery, was originally described in 1995 by Dr. Julian DeLia, and is now being performed in several centers world-wide, including our own. This is the only known form of treatment for twin-to-twin transfusion syndrome where the likely cause of the condition is addressed. In essence, a laser fiber is used, through a very small endoscope (a long telescopic lens) inserted into the uterus, to block all vessels that run from one twin to the other (see figure 3)


Figure 3: The blood vessels that allow transfusion from one twin to the other are closed off by laser. The laser is introduced through a very thin endoscope into the uterus. As a result, each twin gets a separate portion of the placenta, and the twins are now completely separated.

   Figure 4: Laser occlusion of the placental vessels as seen through the endoscope.

Fetal surgery is done under epidural or general anesthesia. A small incision is made on the mother's abdomen and, under ultrasound guidance, a long and thin instrument (less than 1/8 " in diameter) is introduced into the amniotic cavity. This instrument contains an endoscope, which allows the surgeon to directly look into the uterus; and a laser fiber to coagulate, or block, the blood vessels that are seen to cross from one twin to the other. All other blood vessels, that connect the fetus to its own side of the placenta, are left alone.

The procedure has been performed hundreds of times since it was first described. It was initially developed in the United States, but the majority of cases have been performed in Europe. Today, several centers in the U.S. are offering this technique.

The results of laser fetal surgery appear to be better than those of other techniques, when patients are properly selected. Survival of at least one twin has been reported to be 70-80%, and survival of both twins is seen in 30-40% of the pregnancies. Also, the complication rate appears to be much lower. There are much fewer cases of severe heart and brain damage, and the overall complication rate (including a few instances of infection and damage to the membranes) is less than 10%, not including PPROM.

Preterm, premature rupture of the membranes (PPROM) usually leads to premature delivery; it is common in twin gestations (5-15%), particularly if TTTS is present. While decreasing the polyhydramnios (see under Amnioreduction) reduces that risk, it may still occur as a result of the invasive procedure itself. After laser coagulation, the incidence of PPROM may be as high as 15%, particularly if multiple procedures (such as repeated amnioreductions) preceded the endoscopic operation.

Since its first description, the technique has undergone many improvements. For example, the use of special endoscopes that can be custom-curved now allows the treatment of patients with an anterior placenta. The use of magnetic resonance imaging (MRI) and computerized three-dimensional reconstruction allows the surgeons to plan the procedures on a virtual reality model, before the uterus is even entered (figure 5).

Virtual fetuses

Figure 5: Using detailed MRI Images of the fetus (left) and the uterus, a "virtual patient" can be created and manipulated (center), allowing the fetal surgeons to better plan the operation on the placenta (right: green line is the intwertwin membrane on the placental surface).

It is important to note that this is a form of fetal surgery and the most aggressive of the treatment options for TTTS, and that risks, although reduced to a minimum, exist for the mother and the fetuses. While the procedure aims at separating the two blood circulations, communications may still persist after the operation. As a result, one fetus's demise could theoretically still affect the other (although that risk is clearly much reduced compared with any of the other treatment options). Complications of TTTS, such as fetal death, brain damage, limb necrosis and others have been reported even when laser therapy was performed. It is our belief that most of these represented previously undiagnosed abnormalities (undiagnosed, perhaps, because of limitations on the ability of ultrasound to detect evolving problems in their early stages); nevertheless, the possibility also exists that the treatment may not be successful and the TTTS may progress.

Until recently, it had not yet been demonstrated in a scientific way that endoscopic laser ablation of placental vessels was clearly superior to amnioreduction. Therefore, our center participated in a randomized, controlled trial involving many institutions throughout several continents, and administered by Eurofoetus. Patients who qualify were offered to enroll in the study, which aimed to compare serial amnioreduction with endoscopic laser surgery. The study showed that laser ablation was superior to amnioreduction for advanced stage TTTS. This is the first time a randomized controlled study has been performed for fetal treatment of TTTS. It is also the first study to demonstrate that laser surgery is superior to amnioreduction.

For more information regarding the recently completed Eurofoetus study, please contact us or go to the Eurofoetus web site, where a full description of the trial protocol is available. The complete results of the study were published in 2004 in the New England Journal of Medicine. Click here to read the press release an further information regarding this study.

4. Amniotic septostomy.

A few centers have, in the past, offered an alternative procedure ,whereby one or more small holes are created between the two amniotic cavities, in the hope that fluid from the recipient twin (with polyhydramnios) will flow toward the donor twin (who has oligohydramnios). The reasoning is that this way, both twins will have a normal amount of amniotic fluid. Some early results appeared promising, but this technique has since been abandoned.





Survival of both twins



Survival of at least 1 twin



Maternal complications      

 PPROM (5-15%)

 PPROM (5-15%) [1]

 PPROM (5-15%)

 Infection [1]



Fetal complications:      
Neurologic/Cardiac damage



Limb necrosis



Organ damage/absence




[1] Risk increases with increasing number of amnioreductions

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