We all have several different markers, called antigens, in our red blood cells. The bodies of people who don't have certain antigens often make antibodies, or large molecules that can attack and destroy antigens that are foreign to them. One of the antigens in our red blood cells is the Rh factor. In the United States, about 85 percent of the population is Rh positive and 15 Rh-negative. People who are Rh negative and are exposed to Rh positive red cells can become sensitized and start making antibodies that attack and destroy the Rh positive cells which are foreign to them.
Sensitization can happen, for example, if someone receives the wrong kind of blood in transfusion.
The chance of receiving the wrong blood type is remote, because blood banks are very careful in matching the blood types. During a normal pregnancy, an Rh-negative mother can become sensitized at any time, because the fetal red cells regularly enter her circulation. If she has a miscarriage Rh-sensitization doesn't usually create a problem in a first pregnancy. These antibodies can cross the placenta and attack the fetus' red cells, often leading to destruction of the cells and subsequent fetal anemia. As a result the fetal heart may not be able to keep up with the blood volume, and the fetus can develop heart failure and accumulate excess fluid in its body.