When a woman with Rh negative
blood group is pregnant with a baby
with Rh positive blood group, she
might be exposed to Rh positive
blood during any of the following
events:

Any bleeding during pregnancy

Miscarriage or ectopic pregnancy(when the site of pregnancy is outside of the uterus)

Any tests such as amniocentesis (when amniotic fluid is taken through a syringe)

During the delivery Once the mother is exposed, she produces antibodies against the Rh

antigen and becomes ‘sensitised’.

These events are referred to as
sensitizing events.

While the first baby is not much
affected by this, in the subsequent
pregnancy, if the baby is Rh positive, complications can occur.
The antibodies that the mother had
developed during her first pregnancy get passed on to the baby and start attacking the red blood cells(RBCs) of the baby that have the Rh factor. RBCs break down and this is called hemolysis and this can cause anemia and heart failure among other problems.