During pregnancy, the blood circulation through the foetus (baby) and the placenta is totally separate to the blood circulation in the mother. The circulation in the foetus and placenta is maintained by the beating of the foetal heart. It is not unusual for a small amount of blood to leak from the circulation of the foetus into the circulation of the mother, particularly during delivery. This is known as foetomaternal haemorrhage.
Normally this haemorrhage causes no problems, but if the father’s blood is rhesus positive, it is possible for the foetus to also have rhesus positive blood, and if this leaks into the circulation of a mother who is rhesus negative, antibodies against the foetus blood may develop.
The antibodies in the mother’s blood may return to the blood of the foetus and start to attack and destroy the red blood cells, resulting in haemolytic disease of the newborn (HDN). The antibodies remain in the mother circulation, and although the first pregnancy with a rhesus positive baby is not usually a problem, almost certainly subsequent pregnancies will be.
For this reason, all women who are rhesus negative are given an injection of anti-D (rhesus D immunoglobulin) to prevent the formation of antibodies against the Rhesus factor. The injection is given twice during the pregnancy (usually at 28 and 34 weeks) or immediately after birth, or earlier in pregnancy if an amniocentesis is performed, or after a miscarriage, termination of pregnancy or ectopic pregnancy. A test for the presence of anti-D antibodies is usually performed before the injection of anti-D is given.