In about 70% of monovular (identical) twin pregnancies the two foetuses share a placenta. This is known as a monochorionic twin pregnancy. This brochure is for patients who are pregnant with monochorionic twins.
Monochorionic twin pregnancies carry a higher risk of complications than biovular (fraternal) twin pregnancies or single-foetus pregnancies. Complications here mean a greater risk of growth retardation or premature birth, but especially the risk of the so-called Twin-to-Twin Transfusion Syndrome (TTTS). This is a disorder which only can occur in monochorionic twin pregnancies and which can be detected with an ultrasound. TTTS treatment in the Netherlands is only possible in the Leids Universitair Medisch Centrum.
This brochure explains what TTTS is, how it is diagnosed and what treatment is available.
There are two types of twins: monovular (identical) and biovular (fraternal). The chance of a biovular-twin pregnancy is about 1.5%. Biovular twins always have a placenta and amniotic sac each and are referred to as dichorionic twins. The chance of a monovular-twin pregnancy is about 0.5%. Some monovular twins also have a placenta and an amniotic sac each (and are therefore referred to as dichorionic). Most monovular twins (70%), however, share a placenta but do have an amniotic sac each. They are referred to as monochorionic twins.
Checking the thickness of the membrane between the foetuses (the septum) with an ultrasound in the first months of the pregnancy can determine whether the twin pregnancy is monochorionic or dichorionic (figures 1a and b).
Figure 1a. Ultrasound images of a monochorionic (above) and a dichorionic twin pregnancy (below) at five weeks
Figure 1b. Ultrasound images of a monochorionic (above) and a dichorionic twin pregnancy (below) (ultrasound images: www.fetalmedicine.org)
Twin pregnancies, monovular as well a biovular, carry a greater risk of complications than single-foetus pregnancies. One complication however only occurs in monochorionic twins and is called the Twin-to-Twin Transfusion Syndrome (TTTS). In a monochorionic twin pregnancy the risk of TTTS is about 15% (1 in 7).
What is the Twin-to-Twin Transfusion Syndrome (TTTS)?
The cause of TTTS lies in the placenta. In most monochorionic twin pregnancies each foetus has an amniotic sac of its own, but they do share a placenta. There are blood vessel connections between both foetuses on the shared placenta. Via these blood vessels the blood circulation of both foetuses are connected. One foetus gives blood to the other and vice versa. This is a normal situation if there is a balance between the foetuses. Sometimes however a problem occurs when the blood flow in the blood vessels across the placenta is mostly in one direction. One foetus (the donor) gives blood transfusions to the other foetus (the recipient) but receives little in return. The result for the donor foetus is a shortage of blood resulting in a decrease of its urine production that can later even cease altogether, depleting the amniotic fluid. The membrane of the amniotic sac closes tightly around the donor foetus and can hardly be seen on an ultrasound. This is also called a “stuck” twin (figure 2).
Figure 2. Ultrasound image of a stuck twin.
The recipient foetus on the other hand receives a lot of blood and produces more and more urine. It therefore suffers from excessive amniotic fluid in its sac (figure 3). As a result the stomach of the mother swells fast. Excessive amniotic fluid may cause her waters to break and contractions may occur. By this time the foetuses are already seriously ill because of an advanced stage of TTTS and there is a high risk of premature, and fatal, birth.
Figure 3. Diagram of recipient (A) and donor foetuses (B)
When can TTTS occur during the pregnancy?
In principle TTTS can occur at any time during pregnancy. However, research shows that the risk is greatest between 16 and 26 weeks.
What are the symptoms of TTTS?
Women suffering from TTTS often notice that their stomachs swell rapidly in a short time (a number of days to weeks). This is an alarm signal and a reason to immediately consult a gynaecologist. This may be coupled with a very tight feeling and a hard stomach. The rapid swelling of the stomach is the only sign of TTTS for the mother and is caused by the excessive quantity of amniotic fluid of the recipient foetus.
How is TTTS diagnosed?
TTTS is diagnosed by means of an ultrasound assessment. A number of things need to be checked:
Amount of amniotic fluid of each foetus
The donor foetus (which gives away part of its blood) produces increasingly less urine and has therefore less amniotic fluid than the recipient foetus (which receives the blood). Often the donor foetus has no amniotic fluid left at all, and the recipient foetus has an excessive amount.
The donor foetus produces increasingly less urine, with the result that its bladder gets emptier and emptier and in the end is totally empty, the recipient foetus on the other hand produces more and more and its bladder gets increasingly full.
The donor foetus gives away part of its blood to the recipient foetus. This means that the donor foetus has too little blood and the recipient foetus has too much blood. Both foetuses can suffer illness as a result. This can be determined by measuring the speed and pattern of the blood flow in various blood vessels of the foetuses.
Accumulation of fluid
It may happen that the heart of the recipient foetus can no longer properly pump the blood around the body. This influences the blood flow in the foetal blood vessels. There might also be an accumulation of fluid within the foetus. This can be detected with an ultrasound assessment.
The ultrasound is used to check fluid accumulation and to decide whether there is a case of TTTS. If TTTS is diagnosed, a certain categorisation is applied to indicate how advanced it is and its effects on the foetuses. Five consecutive stages are identified, stage 1 being the mildest form of TTTS (only too little amniotic fluid in one sac and too much in the other, without the foetuses being seriously ill) and stage 5 being the most severe stage (one or both foetuses have already died in the womb from TTTS).
Is there a treatment for TTTS?
There are several treatments for TTTS which can be applied depending on the stage of the pregnancy and the severity of the TTTS.
This type of treatment entails that the cause of the TTTS (the blood vessel connections on the placenta) is tackled. It can be compared with laparoscopic surgery. A tiny (2 mm) camera with a laser thread (1 mm) is placed in the amniotic sac of the recipient foetus through the mother’s abdomen (see figure 4). In this way the blood vessels on the placenta can be viewed (see figure 5). All blood vessels between the donor foetus and the recipient foetus (causing the TTTS) are closed off with the laser so as to divide the shared placenta. In this way, the shared placenta is artificially divided in two separate parts. The donor and the recipient now have their own placenta section and their blood flow is no longer connected. The excess amniotic fluid is also tapped. After receiving this treatment the mother is kept for observation in the Department of Obstetrics for 24 hours. During the treatment the mother receives, if necessary, intravenous pain relief and tranquilizers and the skin is anaesthetised locally.
Figure 4. Longitudinal section of a womb with TTTS. The shaft with the camera, laser thread and rinsing system has been inserted through the abdomen of the mother into the amniotic fluid. (Courtesy Prof. Dr. J. Deprest, Leuven)
Figure 5. Photograph of a placenta (after the birth) with clearly visible the blood vessel connection between donor and recipient foetuses.
There is alternative treatment which aims at combatting the symptoms of TTTS. This involves draining the excess amniotic fluid from the amniotic sac of the recipient foetus by means of amniocentesis (amniodrainage). A thin needle (0.9 mm) is stuck through the abdomen of the mother into the amniotic sac of the recipient foetus. The amniotic fluid is then drained from the abdomen through a small tube attached to the needle. The stomach of the mother gets less tight and the risk of premature birth diminishes. However, this does not eliminate the cause of the problem and the quantity of amniotic fluid of the recipient foetus may increase again within a short time, therefore this method has to be repeated several times.
Generally, one of these two treatments is followed, but there are also three alternative treatments possible.
Umbilical cord coagulation
One of the two foetuses may be gravely ill as a result of TTTS, whereas the other is in good health. If the gravely ill foetus dies in the womb of the mother, this is dangerous for the other (healthy) foetus. When one foetus dies a lot of blood of the healthy foetus may flow via the joined blood vessel connections to the deceased foetus. As a result, the healthy foetus may also die or suffer serious damage. To avoid this the umbilical cord of the seriously ill foetus can be closed off (umbilical cord coagulation). This will cause the ill foetus to die, however, without a large blood flow from the healthy foetus to the dead one. This gives the healthy foetus the best chance to survive.
Sometimes during laparoscopic surgery it becomes clear that laser treatment is not possible (for instance because the donor foetus obstructs the view to the blood vessel connections and therefore it is not possible to use laser treatment). In this scenario one may opt for umbilical cord coagulation of one foetus in order to save the other.
Termination of the pregnancy
Sometimes both foetuses are so severely affected with TTTS that survival without a disability is negligible. In that case it may be decided to terminate the pregnancy (provided that the pregnancy is not beyond 24 weeks).
Finally, there is the additional option to do nothing and let nature run its course. In stage 1 - not too much amniotic fluid and a stable situation - sometimes things improve naturally.
What is the prognosis after treatment?
A TTTS of stage 2 or higher left untreated and left to run its course, almost invariably ends with an extremely premature birth. Quite often both foetuses die.
Amniodrainage entails a minor risk that the waters break (about 1% per treatment). The survival rate of foetuses after repeated amniodrainage is 66%, with a 52% chance that both foetuses survive, 18% that one foetus survives and 30% that they both die. The average length of the pregnancy before birth is 29 weeks. The risk of brain damage for the surviving babies is 16%.
Laser treatment entails a greater risk that the waters break (10%). The instrument used to insert the laser is 3 mm thick and has to go through the membrane in order to enter the amniotic sac of the recipient donor. Besides there is a 10% chance that not all blood vessel connections can be seen and therefore cannot be lasered. In this case it is possible that the TTTS is not cured after the laser treatment. In very rare cases it may also happen that the TTTS reverses after a few weeks and that the former donor becomes the recipient and vice versa.
If the surgery is a technical success (i.e. all vessel connections can be detected and lasered and the waters do not break) it may happen that one or both foetuses still die during the pregnancy as a result of unequal division of the placenta.
The survival rate of foetuses after laser treatment is 75%, with a 54% chance that both foetuses survive, 27% that one foetus survives and 19% that neither of the foetuses survives. These figures are based on the greatest number of laser treatments carried out in the world and they confirm the results of LUMC laser treatments. The average pregnancy duration is 35 weeks. The risk of brain damage for surviving children is 7%.
The risk that the waters break as a result of an umbilical cord coagulation are similar to the risks of laser treatment (10%). The survival rate of the healthy foetus after umbilical cord coagulation of the ill foetus is 85%.
The deceased foetus remains in the womb during the rest of the pregnancy. This does not have any detrimental effects for the mother or the healthy foetus. After the birth of the remaining child, the deceased foetus usually comes out together with the placenta. Often it will be difficult to recognize this foetus, because the other child has used all available space in the womb to grow and has therefore deformed the dead foetus and pushed it into the womb’s membrane.
Are there any detrimental effects for the mother?
Laser treatment and umbilical cord coagulation carry the same risk of complications for the mother as laparoscopic surgery (less than 1%), Amniodrainage has an even smaller risk for the mother.
Should you have any questions after reading this information please feel free to contact one of the specialist doctor-sonographers or gynaecologists.
Outpatient Department of Obstetrics and Prenatal Diagnostics
Telephone: 071-526 3325 (during business hours)
Telephone: 071-526 2853 (emergencies, available 24 hours per day)
Leids Universitair Medisch Centrum
2300 RC Leiden
Department of Obstetrics of the LUMC
www.nvom.net (Nederlandse Vereniging Ouders van Meerlingen, the Dutch Association for Parents of Multiple Births, DAPMB)