The Rh factor is a protein found on the surface of red blood cells. If your blood cells have this protein, you are Rh positive, and if your blood cells do not have this protein, you are Rh negative. The "+" or "–" you might see after your blood type refers to Rh positive or Rh negative. Rh positive is much more common than Rh negative. Having an Rh negative blood type is not an illness and usually does not affect your health. However, it can affect pregnancy. During pregnancy, problems can occur if you are Rh negative and your fetus is Rh positive, which is called Rh incompatibility. If the mothers blood and the babys blood mix during birth, the mothers body might begin making antibodies against the Rh protein, which can lead to severe fetal anemia in a future pregnancy. Therefore, it is important to determine the Rh factor for a mother and unborn child during pregnancy. Rh factor incompatibility only becomes an issue during pregnancy when the mother is Rh-negative and the fetus is Rh-positive. Knowing your Rh status is important during pregnancy because it can prevent potential complications.