In identical twins, a single egg splits shortly after conception into two identical embryos.
Doctors believe if splitting occurs within three days of conception, then twins have separate placentas.
If splitting takes place after this, the twins share a placenta. In such cases, blood vessels form a link between the developing babies.
In babies with the syndrome it is thought these vessels are imbalanced which means blood from one twin is transferred via the placenta into another.
The recipient twin is usually larger from being overloaded with blood, and the heart must work harder. This can lead to heart failure. The donor twin is usually smaller and can become anaemic.
Eight out of ten untreated cases result in death. It can also cause disability in twins that manage to survive.