An event which can occur in same sex, identical twins who share a placenta. It results when one twin's blood transfuses into the other twin through their shared placental vessels. The risks of this situation include major rapid changes in blood pressure in one or both and possible severe anaemia or its opposite, too much blood. These fluctuations in blood supply, especially to the brain and lungs can lead to stillbirth or survival with brain damage and complications such as CEREBRAL PALSY.