What is Twin to Twin Transfusion Syndrome?
This will be a pregnancy where identical twins (of the same sex) share one placenta, and have a thin membrane separating them, each in his or her own amniotic sac. If you are pregnant with twins please find out from your Obstetrician whether you have a monochorionic-diamniotic twin pregnancy as TTTS mostly occurs in these types of pregnancies.
TTTS can be explained as the disproportionate flow of blood from one twin to the other though connecting blood vessels in the placenta that the babies share. How TTTS occurs is not known, so it cannot be prevented.
TTTS is a disease of the placenta and the babies are usually normal. There is nothing wrong with the babies, only with how they are being nourished by their life support system - the placenta. Depending on the stage of TTTS diagnosed one or both babies will be effected by the disproportionate flow of blood in various ways.
The babies will differ slightly in size, the larger baby (twin A) usually known as the recipient twin and the smaller baby (twin B) the donor twin. The donor is smaller due to receiving less blood and nourishment through the umbilical cord and the recipient is larger from receiving too much blood.
Blood flows from the smaller donor baby through the umbilical cord, through the connecting blood vessels in the placenta and then through to the larger recipient baby. The umbilical cord of the donor baby is usually smaller than that of the recipient. The flow of blood to the donor is less than that received by the recipient. The smaller baby usually works harder pumping blood for himself and the recipient.
TTTS is usually identified at a sonar where a sonographer will perform about a one hour long sonar or scan of both babies and check various criteria.
The separating membrane between the babies is sometimes difficult to identify on a sonar and they should take care not to miss identifying the membranes presence in determining a monochorionic-diamniotic twin pregnancy. The other factor is one placenta shared by both babies, and again they should carefully evaluate that only one placenta is present and not two that have fused together.
The only way to monitor a TTTS diagnosed pregnancy, and identify the level of severity is by sonar. Both babies will have to be checked regularly for signs of stress.
HOW THE TWINS ARE AFFECTED BY TTTS DURING PREGNANCY
The recipient twin, due to this extra flow of blood, will urinate more causing polyhydramnios (an excessive amount of amniotic fluid in his sac). The bladder of the baby can usually be seen on the sonar as full and the baby's abdomen circumference may be larger than for the expected gestational age. As the baby grows, so does the amount of amniotic fluid. The amniotic fluid levels can be measured and can show pockets or volumes of up to 12 cm deep.
The donor twin, the smaller baby, does not get enough blood and may die from severe anemia. The bladder of the donor twin will be much smaller or non existent on the sonar. The non existent bladder will indicate a more severe stage of TTTS. Because the donor twin is receiving less blood he will not urinate as frequently causing oligohydramnios (too little amniotic fluid in his sac).
The donor twin can also be seen as stuck to the uterine wall as his membranes from his sac wrap tighter around him as his amniotic fluid diminishes. The donor twin is less active due to the restricted space and the mother will find that this baby will kick less than the recipient twin.
The donor's umbilical cord may also be under pressure or be pressured by the baby against the uterine wall or elsewhere, worsened by the presence of too little amniotic fluid and the wrapping of the membranes around the babies body. The flow of blood through the umbilical cord to the baby is even further diminished in this case.
The donor twin is at a higher risk during pregnancy of dying in-utero from anemia, causing a transfusion of blood to the recipient twin at that time, who then will die from a heart attack due to the excessive amount of blood transfused causing a strain on his cardiovascular system.
The donor twin can be as much as 25% or more smaller than the recipient twin.
The radiologist when taking the sonar should be careful in the diagnoses of polyhydramnios in both sacs due to the membrane not being visible. On closer inspection the radiologist will see the membrane is not where it is expected to be, glad-wrapped around the baby! The radiologist can also request the mother turn on her side during the examination and identify that the donor twin is still stuck in the same position (stuck twin) on the sonar, evidencing TTTS diagnosis. The stuck twin also seems to adopt a fetal position and the sex may be difficult to tell as the baby always seems this way presented at each sonar.
HOW THE MOTHER IS AFFECTED DURING A TTTS PREGNANCY
The TTTS mother will show an abnormally large abdomen due to the excessive amniotic fluid, will experience heaviness or pain on the pelvic region from the weight, and will urinate more frequently from the pressure on her bladder. Polyhydramnios can cause a 20 week pregnancy to look like a full term pregnancy! Sore ribs, difficulty in breathing...
Besides the discomfort that is experienced, there is an increased risk of premature rupture of the membranes and premature delivery of the twins due to polyhydramnios. The risk of premature delivery due to early rupture of the membranes is that the babies may not have reached a viable gestational age and will be either stillborn or if before 23 weeks gestation will not be resuscitated at birth due to the high mortality rate amongst these tiny babies and the very high risk of disability should they survive. The mother's cervix should be regularly checked by the Obstetrician at all prenatal visits to determine when and if early rupture of the membranes is anticipated. In this case the mother should be put on immediate bed rest to take the pressure off the cervix.
The TTTS pregnancy is very high risk and the mother should be on bed rest where possible especially when polyhydramnios has been diagnosed.
The rates for survival of both twins suffering from TTTS is 10% if nothing is done to assist them during the pregnancy. Medical technology today can increase the chances of survival for both babies in a TTTS pregnancy once diagnosed. The problem is that many pregnancy's go undiagnosed resulting in a higher mortality rate.
TTTS only occurs in 10-15% of all monochorionic-diamniotic twin pregnancies. Being such a rare condition, medical research has been somewhat neglected in this area and not much material is available on TTTS. The first step in saving these babies is being diagnosed with TTTS in the first place.
Awareness of TTTS needs to be amongst Obstetricians and Gynaecologists so that the detection early on in a TTTS pregnancy can offer maximum medical assistance or treatment in time to make a difference.
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