Just as there are different major blood groups, such as type A and type B, there also is an Rh factor. The Rh factor is a protein that can be present on the surface of red blood cells. Most people have the Rh factor—they are Rh positive. Others do not have the Rh factor—they are Rh negative.
The Rh factor is inherited—passed down through parents’ genes to their children. If the mother is Rh negative and the father is Rh positive, the fetus can inherit the Rh gene from the father and could be either Rh positive or Rh negative. If the mother and father are both Rh negative, the baby also will be Rh negative.
The Rh factor can cause problems if you are Rh negative and your fetus is Rh positive. This is called Rh incompatibility. These problems usually do not occur in a first pregnancy, but they can occur in a later pregnancy. When an Rh-negative mother’s blood comes into contact with blood from her Rh-positive fetus, it causes the Rh-negative mother to make antibodies against the Rh factor. These antibodies attack the Rh factor as if it were a harmful substance. A person with Rh-negative blood who makes Rh antibodies is called “Rh sensitized.”
Problems during pregnancy can occur when Rh antibodies from an Rh-sensitized woman cross the placenta and attack the blood of an Rh-positive fetus. The Rh antibodies destroy some of the fetal red blood cells. This causes hemolytic anemia, where red blood cells are destroyed faster than the body can replace them.
Red blood cells carry oxygen to all parts of the body. Without enough red blood cells, the fetus will not get enough oxygen. Hemolytic anemia can lead to serious illness. Severe hemolytic anemia may even be fatal to the fetus.