Rh incompatibility is when a mother is Rh negative and her baby is Rh positive, which can cause problems during pregnancy.

If the baby’s Rh positive blood cells enter the mother’s circulation, her immune system may view them as “foreign” — and, in a normal immune response, mobilize armies of antibodies to attack these unfamiliar cells.

Since red blood cells carry oxygen, this type of attack can make it hard for a baby to get the amount of oxygen it needs. That can potentially cause a serious type of anaemia and other health problems.  Rh incompatibility only becomes a problem if you and your baby’s blood mix, most likely during birth. At that point, the antibodies your body makes cannot harm your baby.