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 you're pregnant, blood from your baby can cross into your bloodstream, especially during delivery. If you're Rh-negative and your baby is Rh-positive, your body will react to the baby's blood as a foreign substance.
Your body will create antibodies (proteins) against the baby's Rh-positive blood. These antibodies can cross the placenta and attack the baby's red blood cells. This can lead to hemolytic anemia in the baby.
Rh incompatibility usually doesn't cause problems during a first pregnancy. The baby often is born before many of the antibodies develop.
However, once you've 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.