Rhesus haemolytic disease is a condition that usually only occurs for babies of women with a 'negative' blood group, who have had a previous pregnancy where the baby's blood group was 'positive' (or if the woman has received blood products in the past, such as a blood transfusion).
At birth, it is normal for a small amount of the baby's blood to cross over into the mother's circulation entering her blood stream, usually as the placenta separates. This 'crossover' of blood is also possible in a few other circumstances such as miscarriage, (including early miscarriages that you may not of even been aware of - although this is rare), termination of a pregnancy, heavy bleeding late in pregnancy or during
amniocentesis,
CVS or
ECV procedures.
If the baby's blood is a 'positive' blood group and it passes to their mother who has a 'negative' blood group, it is possible for this small amount of blood to set up a reaction in the mother's blood (about a 10 to 20% chance).
NOTE: If the mother has a 'positive' blood group and the baby has a 'negative' blood group, no reaction will occur because negative blood can be accepted by (or mixed with) all blood groups - often referred to as the 'universal donor' type of blood.
The 'reaction' (also known as 'alloimmunisation' or 'isoimmunisation') that can occur is the formation of blood 'antibodies' or 'agglutinins'. This reaction is a normal response in the human body, used to 'fight' substances that our bodies interpret as being 'foreign invaders' to our system. The antibodies form in the mother's blood in an effort to deal with 'getting rid' of the 'foreign' baby's blood. A woman who has formed antibodies is often referred to as being 'sensitised'. The 'reaction' is similar to the one we intentionally produce when we immunise our bodies against disease.
The formation of antibodies does not affect the health of the woman.