Twin twin transfusion syndrome

TTTS is a condition that only occurs in twins that share a single placenta.

What is twin twin transfusion syndrome (TTTS)?

TTTS is a condition that only occurs in twins where there is a shared placenta (monochorionic). In shared placenta, there are blood vessels that connect the twins’ circulations. Most of the time (85-90%), blood flows in both directions with no overall change to each twin’s circulation. 

However, in 10-15% of these pregnancies, the overall blood flow between the twins is unbalanced. This leads to one twin becoming a blood 'donor' to the other 'recipient' twin. When this happens, the donor twin becomes anaemic (with a low blood count), and stops making urine. A donor twin becomes much smaller that the recipient twin and has very little amniotic fluid in its sac.

The 'recipient' twin has an overloaded blood circulation which can lead to heart strain. The recipient twin also produces increased amounts of urine leading to excessive amniotic fluid. This excessive fluid can put the woman at risk of preterm labour and early delivery of both twins.

How is TTTS diagnosed?

TTTS can occur at any stage of the pregnancy and is diagnosed by ultrasound. It is therefore recommended that monochorionic twin pregnancies have fortnightly ultrasounds from around 14-16 weeks’ gestation until birth. The ultrasound features of TTTS include:

  • An imbalance of amniotic fluid volume around each twin, with the recipient twin having too much fluid ("polyhydramnios") and the donor twin having too little ("oligohydramnios")
  • The donor twin may have an empty bladder, which is a sign of reduced urine production 
  • The blood flow patterns ("Dopplers") through the umbilical cord or inside each twin may be abnormal
  • The size of the donor twin is usually much smaller than the recipient twin

FAQ on treatment of TTTS

Before the introduction of laser treatment for TTTS, severe TTTS resulted in a greater than 90% loss of the pregnancy. Treatment options depend on the gestation at diagnosis. In general, laser is offered between 18 and 26 weeks’ gestation. Prior to 18 weeks’ laser is challenging with increased risks of complications, so is rarely offered.
After 26 weeks’ survival in a neonatal intensive care nursery is generally excellent and mostly conservative treatments such as draining of amniotic fluid is offered rather than laser. If the condition is progressive, delivery can be offered.

If TTTS is diagnosed at any stage of a pregnancy, referral to the Victorian Fetal Therapy Service (VFTS) is recommended so that the TTTS can be assessed. VFTS is a collaboration between the Mercy Hospital for Women, Monash Medical Centre and the Royal Women’s Hospital, with a team of surgeons from each of these sites. Once the TTTS has been assessed by one of the VFTS team, appropriateness for laser surgery can be determined and organised by this surgeon. 

All twin laser surgery in Victoria takes place at Monash Medical Centre, as the equipment and laser safety expertise is located at this site. The VFTS surgeons travel to Monash Medical Centre in Clayton for the surgery and at least one doctor from Mercy Perinatal will be present.

Fetoscopic laser surgery involves a procedure under local anaesthetic, whereby an operative endoscope (fetoscope) is inserted through a mother’s abdomen, through the wall of the uterus, and into the sac of the recipient twin. The blood vessels connecting the twins that are causing the TTTS are identified. A laser fibre is passed down the fetoscope, and the laser is fired at these connections to close them. At the end of the procedure, the excessive amniotic fluid is drained from the recipient twin’s sac to reduce the risk of preterm birth or ruptured membranes. Patients are usually admitted overnight and discharged back to their referring hospital for ongoing follow-up.

Video of laser surgery

A short video from the Johns Hopkins Centre for Fetal Therapy in the United States showing the surgical view from the fetoscopic camera can be viewed here.

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