In one in 50 pregnancies there are two fetuses. In 80 per cent of
twin pregnancies the two fetuses live in their own sac and may have their own placenta
(dichorionic). In 20 per cent of twins the fetuses also have their own sac but they share
the same placenta (monochorionic).
- Twin to twin transfusion syndrome
In one third of monochorionic twin pregnancies there is an
imbalance in the net flow of blood across the placenta vascular communications from one
fetus (the donor) to the other (the recipient). This condition is called twin to twin
transfusion syndrome (TTS). In half of the cases the condition is mild to moderate and
without any treatment usually both babies survive. However, the pregnancy needs to be
monitored closely and delivery is undertaken at 32-34 weeks. In the other half (or 15
percent of all monochorionic twins) the condition is severe and both babies are at very
high risk of death or handicap.
Severe disease often becomes apparent at 18 to 22 weeks of
pregnancy, with their mother complaining of a sudden increase in the size of her abdomen
associated with extreme discomfort, and occasionally respiratory distress.
In severe TTS the donor fetus tries to compensate for the blood
loss by stopping urine production. Since all the amniotic fluid is urine, a lack of
urination causes a decrease and eventually absence of amniotic fluid. This fetus gets
'stuck' in and immobile at the head of the placenta where it is held fixed by the
collapsed amniotic sac. Without any treatment this donor twin is at high risk of death or
handicap due to deficiency in oxygen, nutrients and blood.
The recipient twin tries to compensate for the extra blood it
receives by producing more urine. This causes a big increase in the amount of amniotic
fluid (polyhydramnios). This distends the uterus leading to miscarriage or very severe
preterm delivery. Eventually the recipient twin develops congestive heart failure.
- Options for management of the syndrome
There are essentially four options in the management of severe
- No treatment. In more than 90 per cent of cases the pregnancy will
end in miscarriage or very severe preterm delivery; both babies will die and the few that
survive have a high risk of being handicapped.
- Termination of the pregnancy.
- Amniocentesis and drainage of large amounts of amniotic fluid. This
treatment usually needs to be repeated every one to two weeks. There is controversy
concerning the effectiveness of this therapy with the reported survival varying from 40
per cent to 80 per cent. However, there is agreement that about 25 per cent of the
survivors are handicapped.
- Laser surgery.
This method involves the use of laser to block the placental blood
vessels that connect the circulations of the two fetuses.
Detailed ultrasound examination is first performed to choose the
appropriate site of entry on the maternal abdomen to avoid injury to the placenta or
fetuses and to allow access to the suspected area of vascular communications.
An injection of local anaesthetic is given and under continuous
ultrasound visualisation, a thin (2.5 millimetre diameter) fetoscope is introduced into
the amniotic cavity of the recipient twin.
A combination of ultrasonographic and direct vision is used to
examine systematically the surface of the placenta along the whole length of the
inter-twin membrane and identify the crossing of vessels, which are coagulated by the use
of laser. The total procedure usually takes 30 to 45 minutes to complete.
In one third of cases both babies survive, in one third only one
baby survives, and in one third both babies die. The chance of a handicap in the babies
that survive is about five per cent.
- What to expect after the operation
In the first couple of days you may experience some abdominal
discomfort, period-like pain or a little bleeding. The symptoms are relatively common and
in the vast majority of cases the pregnancy continues without any problems. You may find
it helpful to take simple painkillers like paracetamol (two tablets every four to six
hours to a maximum of eight tablets in 24 hours). If there is a lot of pain or bleeding or
if you develop a temperature, please contact your hospital immediately.
If the operation is successful and one or both fetuses survives it
is necessary to monitor the pregnancy with scans every one to two weeks to confirm that
the fetuses are developing normally.