A difference in blood type between a pregnant woman and her baby causes Rh incompatibility. The condition occurs if a woman is Rh-negative and her baby is Rh-positive. When youre pregnant, blood from your baby can cross into your bloodstream, especially during delivery. If youre Rh-negative and your baby is Rh-positive, your body will react to the babys blood as a foreign substance. Your body will create antibodies (proteins) against the babys Rh-positive blood. These antibodies can cross the placenta and attack the babys red blood cells. This can lead to hemolytic anemia in the baby. Rh incompatibility usually doesnt cause problems during a first pregnancy. The baby often is born before many of the antibodies develop. However, once youve formed Rh antibodies, they remain in your body. Thus, the condition is more likely to cause problems in second or later pregnancies (if the baby is Rh-positive). With each pregnancy, your body continues to make Rh antibodies. As a result, each Rh-positive baby you conceive becomes more at risk for serious problems, such as severe hemolytic anemia. 
What causes Rh Incompatibility ?