What is Twin to Twin Transfusion Syndrome (TTTS)?

Also sometimes known as 'FFTS' (Feto-Fetal Transfusion Syndrome), and as 'TOPS' (Twin Oligohydramnios-Polyhydramnios Sequence), TTTS is a disease of identical twin fetuses caused by abnormal connecting blood vessels in the twins' placenta, resulting in an imbalanced flow of blood from one twin to another. The implications of this are very serious for the survival and health of both twins. Once thought to be extremely rare, TTTS is now believed to affect as many as 1 in 1000 pregnancies.

A useful explanation first about what happens normally following conception. The following paragraph is from the British Medical Association's booklet entitled "Understanding Pregnancy".

"Your baby starts to grow from the moment of fertilisation. But its real development begins about 5 days later, when the fertilised egg, now a cluster of over 100 cells,reaches the uterus and becomes embedded in its spongy lining. Fingers from the outer cells of the cluster start to burrow into the lining like roots, linking with the mother's blood supply. These will form the placenta, which will supply all the nutrients the baby needs and carry away all its waste products. Some of these outer cells develop into the umbilical cord, a rope of blood vessels which link the baby to the placenta, and others form membranes which protect the baby."

According to research figures, up to a third of twin & multiple pregnancies involve identical (correctly known as 'monozygotic') twins. Two-thirds of these share the same placenta, and TTTS affects from 5-25% of these. Monozygotic twin pregnancies occur when one group of developing cells (known as a 'blastocyst'), from one fertilised egg, splits shortly after conception to create two identical embryos. In monozygotic twins, it is believed that the later the egg splits the greater the chance of conditions such as TTTS. Nobody quite knows why an egg/blastocyst splits to make monozygotic twins or understands the factors influencing the timing of the splitting. It is also not fully understood why some develop TTTS.

In Chapter 15 - 'Multiple Pregnancy' in 'Dewhurst's Textbook of Obstetrics & Gyneacology', Professor Nicholas Fisk of Queen Charlotte's & Chelsea hospital in London describes three placental configurations for identical twins. He states that if the splitting occurs within 3 days of fertilisation, the twins will have seperate placentas with around half of these appearing to be 'fused placentas' because they are so close together. If splitting occurs between 4-7 days the twins will share a single placenta and each a separate amniotic sac. After 7 days the twins will have a shared placenta and a shared amniotic sac. Professor Kypros Nicolaides of The Harris Birthright Centre in London adds in his document 'Management of severe twin to twin transfusion syndrome' that splitting after 12 days results in cojoined or 'siamese' twins.

In twins that share a common placenta (known as 'monochorionic' twins), blood vessels within the shared placenta commonly form a "link" between the developing babies. In most cases of monozygotic twins sharing the same placenta these joined vessels can include artery-to-artery, vein-to-vein, and artery-to-vein connections. One theory is that usually these connections balance each other out and cause no ill effect.

In TTTS cases, however, it is believed that the joined vessels are imbalanced or are caused to become imbalanced. Essentially blood from the "donor" twin is transfused through the placenta into the "recipient" twin. The fetuses themselves are normal, the abnormality is in the placenta.

Several theories have been proposed over the years for the pathological causes of the condition. Click here for some of them.

TTTS can kick in at any time during the pregnancy, often as early as 4-5 months. TTTS in its worst form,"Chronic TTTS", occurs well before delivery when the babies are too immature to be delivered and cared for effectively. On the other hand "Acute TTTS" can occur later right up to or during delivery, sometimes causing death or handicaps in the surviving infants.

Acute TTTS can occur in any case of monochorionic twins, who may not have shown any of the clinical symptoms of Chronic TTTS through the pregnancy. The pressures invoked by normal delivery can cause a mass transfusion of blood from one twin to another. Experienced hospitals can combat this by monitoring fetal heart rates during delivery for signs of Acute TTTS. If it is detected, an emergency C-Section will usually be offered.

Chronic TTTS can bring several difficulties.

The recipient twin (often larger) becomes effectively overloaded with the extra blood supply. His or her heart has to work harder to pump the enriched blood around, which can result in heart failure. The recipient also produces far too much amniotic fluid - termed "polyhydramnios", which causes the mother to appear far further on in the pregnancy than she actually is. This excess fluid is not only extremely uncomfortable for the mother, but can often bring about an early delivery due to cervical pressure and ruptured membranes, and by directly stretching and stimulating the uterine muscles to contract. The recipient twin may also develop hydrops or edema (retained fluid) within his or her body, which in turn can lead to respiratory, digestive, heart, or brain defects.

The donor twin (often smaller) becomes anaemic due to not having enough blood supply, and usually has a small amniotic sac (the bag of fluid in which the baby lives). This lack of amniotic fluid is termed "oligohydramnios". The sac is often very close to the developing baby's body, and the baby appears to be almost shrinkwrapped and "stuck" up against the wall of the uterus. When donor babies survive birth, however, they are often the baby that is stronger as their hearts have not had to work so intensely during pregnancy.

If left untreated, TTTS has a mortality rate of over 80% and can be the root cause of severe handicaps for the few survivors..

How is Antenatal TTTS diagnosed?

The mother may well experience the following symptoms from early on and throughout a TTTS pregnancy: Premature contractions, Rapidly expanding tummy, Rapid weight gain, Breathlessness, Tightness of tummy, Incredible pressure on stomach.

The single most important fact that needs to be determined early in a twin pregnancy is whether the twins are monochorionic (sharing the same placenta) or dichorionic.

If monozygotic monochorionic twins are diagnosed ultrasound scans should be performed at regular intervals (weekly, fortnightly, or monthly) by a fetal medicine specialist in an attempt to determine whether or not TTTS is likely to develop.

The onset of TTTS is detected through ultrasound scan, (sonogram), at a local maternity hospital. The tell-tale signs are often different-sized babies, different-sized bladders, and too little fluid in one sac and too much in the other. A special type of scan known as a 'colour doppler flow' may also be performed at some hospitals - this can actually track blood flow through the placenta and umbilical cords to better diagnose the condition. An experienced sonographer, radiologist or fetal medicine specialist with the correct equipment may be able to confirm whether or not twins are identical as early as 12-14 weeks. Later on in the pregnancy it may be more difficult to detect a clear image of the separating membranes and the single placenta. Most maternity hospitals are able to diagnose or at least suspect TTTS, but may be unclear as to available treatment. This is understandable as the treatments for TTTS have not been well publicised within the medical profession up till now.

Professor Geoffrey Machin from the Department of Pathology, Permanente Medical Group, Oakland, California, has worked on twin pregnancy problems in Britain, Canada, and the USA. He has written several papers and textbook chapters on particular problems of monochorionic twins.

Professor Machin writes:

"Although the ultrasound diagnosis of well-advanced antenatal TTTS can be made, it is often already too late to plan and carry out treatment that can be fully effective...

If necessary, expectant parents of twins should insist, from earliest stage of pregnancy, that the chorion status of the fetuses be determined accurately, and that they must be given this information."

Additional and more specific information is available for professionals on the diagnosis of TTTS. Call for details of our publications.

Click here for >Diagnostic Criteria for Medical Professionals

Unfortunately, many cases of TTTS go untreated and babies die due to lack of information supplied to parents.

Treatment is available, and there is now less reason to give up hope.