A woman is at risk when she has a negative Rh factor and her partner has a positive Rh factor. This combination can produce a child who is Rh positive. While the mother's and baby's blood systems are separate there are times when the blood from the baby can enter into the mother's system. This can cause the mother to create antibodies against the Rh factor, thus treating an Rh positive baby like an intruder in her body. If this happens the mother is said to be sensitized.
A sensitized mother's body will make antibodies. These antibodies will then attack an Rh positive baby's blood, causing it to breaking down the red blood cells of the baby and anemia will develop. In severe cases this hemolytic disease can cause illness, brain damage and even death.
Rh immunoglobulin (RhIg) is a blood product given via injection to help the Rh negative mother by "minimizing her reaction to the Rh positive red cells. Reactions to the medication are generally minor, including soreness at the injection sight and sometimes a slight fever.
Since a small number of unsensitized women may have problems with the end of pregnancy, many practitioners recommend that she be given an injection of RhIg (also known as Rhogam) at 28 weeks gestation, to prevent the few cases of sensitization that occur at the end of pregnancy. Each dose of RhIg lasts about 12 weeks. The mother will also be given RhIg within 72 hours of birth if the child is Rh positive. The baby's blood type can be determined easily after birth by cord blood samples.
(got this off of a website, it was pretty easy to understand. hope this helps.)