The Rhesus, or Rh factor is a substance found on the surface of red blood cells. If your blood contains Rh0 (D) molecules, you are considered to be Rh-positive, otherwise as Rh-negative if it is absent.
Rh incompatibility develops when the mother, having a Rh–negative blood type and the father, having a Rh-positive blood type produce a foetus whose blood type is Rh-positive. The Rh-positive blood cells from the foetus could pass into the mother’s bloodstream during pregnancy or at birth, where they would be recognised as a foreign substance, in which case, the mother will produce antibodies against the foetus' blood. If this is the mother’s first pregnancy, she will have little or few of these antibodies and they will be unlikely to affect the foetus, however, there will be a strong chance that future Rh-positive foetus’ will be harmed if the mother is not treated accordingly. The result for the foetus may be jaundice, heart failure, anaemia or even brain damage.
The Rh-negative mother’s blood should be checked every two months for these antibodies, especially after any activity which may cause the mother’s and foetus’ blood to mix, such as after amniocentesis or Chorionic Villus Sampling (CVS), and within the first 72 hours after delivery. At these times, the mother should be given a Rh0(D) immune-globulin injection to destroy these antibodies if they exist. This injection is also necessary, should the pregnancy be terminated through an abortion or miscarriage.
Sources: mdguidlines.com; MedlinePlus