When does maternal isoimmunisation occur?

A woman whose blood type is Rh negative, if the male is Rh positive, can become pregnant with an Rh-positive baby. This situation can trigger the mother to recognise the baby's red blood cells as foreign, as the mother's red blood cells (Rh negative) lack this antigen on their surface.

As a result, the mother's immune system will begin to produce anti-Rh antibodies, a process known as Rh isoimmunisation or Rh sensitisation.