Twin-to-twin transfusion syndrome
Introduction
In the United Kingdom the current twinning rate is approximately 15 per thousand births. Three-quarters of all twins are non-identical (dizygotic), and hence the incidence of identical (monozygotic) twinning is 3 - 5 per thousand pregnancies. Of these two-thirds share a single monochorionic placenta, and it is this subset of twins that is at risk of developing twin-to-twin transfusion syndrome
(TTTS).
What is twin-to-twin transfusion syndrome?
In most, if not all, monochorionic twins, there is sharing of blood between the twins within the shared placenta. In TTTS there is an imbalance in this share, so that one twin (‘the donor’) gradually loses blood volume to its co-twin (‘the recipient’). TTTS affects approximately 15-20% of monochorionic twin pregnancies.
The result of this interfetal transfusion is potentially serious for both twins. The growth of the donor fetus can slow, and, because it is losing blood volume, it does not pass so much urine and hence the amniotic fluid around it drops. Finally, the donor fetus make become ‘stuck’ within its membrane. The recipient may also struggle with the process of receiving too much blood volume. Its heart may become dilated and not function effectively, and to compensate this twin will pass vast quantities of urine. This rapid build up in amniotic fluid is called polyhydramnios and may cause a miscarriage or preterm labour to occur.
The overall process of TTTS can be severe, and can lead to a poor outcome for both twins, depending on how rapidly it develops, and at what stage of pregnancy it occurs.
What can be done about the problem?
Once the diagnosis of TTTS has been made, a detailed assessment of the pregnancy will be made. With ultrasound we will assess the condition of both twins and determine whether anything needs to be done, or whether we can adopt a
‘wait-and-see’ approach. Sadly in the majority of cases, some form of intervention will be required.
The choices of intervention are:
(i) opting to do nothing – some couples opt to allow ‘nature to run its course’. Sadly this often means that a miscarriage or an extreme preterm birth may occur
(ii) early delivery – if the TTTS occurs relatively later in the pregnancy
(beyond 26 weeks), then the option of not attempting treatment inside the womb, but rather to deliver the babies will be discussed. There are risks to being born extremely prematurely, however, the treatment alternative options (discussed below) are also risky and hence some

couples opt for delivery (which will of course stop the disease process as the twins would not longer rely on the shared placenta for their survival).
Delivery would almost certainly be by Caesarean section.
(iii) Amnioreduction – this treatment involves using a needle (inserted into the mother’s womb under local anaesthetic) to drain off the extra amniotic fluid that has collected. It is a very simple technique to treat the problem of the accumulating extra amniotic fluid around the recipient twin. However, because it does not stop the underlying disease process itself this option is often not taken.
(iv) Laser ablation – this treatment involves insertion a fetoscope (a key-hole surgery telescope) into the mother’s womb under local anaesthetic. The shared blood vessels within the placenta that are allowing the imbalanced blood flow to happen, can then treated with a laser.
Recent research has shown that this is the best treatment for moderatesevere TTTS, however it is not without risk (see below).
What are the risks?
To the mother:
Any technique that involves inserting a needle or fetoscope into the womb is painful, although the use of local anaesthetic should minimize this. These is also a small risk of blleding behind the placenta (an ‘abruption’) either with the amnioreduction or laser techniques – although the very nature of polyhydramnios in the first place increases these risks.
To the twins:
Once the condition has become serious enough to consider treatment, then the risks to the twins increases. Research has shown that the laser treatment improves the chances of either twin surviving and reduces the risk of handicap in survivors:
In 4/10 cases both twins will survive
In 4/10 cases only one twin will survive
In 2/10 cases neither twin will survive
In view of these risks couples will also be given the option of termination of pregnancy rather than attempting treatment inside the womb.