The Rh factor is a protein that can be found on the surface of red blood cells. If your blood cells have this protein, you are Rh positive. If your blood cells do not have this protein, you are Rh negative. The "positive" or "negative" part of your blood type, such as O positive or A negative, refers to your Rh status.

What is this test? Rhesus (Rh) typing is used to find out if you have a certain protein called Rh factor on the outer layer of your red blood cells. If you don't have Rh factor in your blood, you are Rh negative. If you do have Rh factor in your blood, you are Rh positive. Most people are Rh positive.

Rh incompatibility occurs when the mother's blood type is Rh negative and her fetus' blood type is Rh positive. Antibodies from an Rh negative mother may enter the blood stream of her unborn Rh positive infant, damaging the red blood cells (RBCs).

Rh incompatibility may not cause problems if you receive treatment before antibodies develop. But left untreated, the condition can lead to dangerous pregnancy complications and fetal conditions such as hemolytic disease of the newborn (HDN). HDN is a blood disorder that affects newborns.