Rhesus (Rh) factor is an inherited protein found on the surface of red blood cells. If your blood has the protein, you're Rh positive. If your blood lacks the protein, you're Rh negative. Rh positive is the most common blood type.

What is the Rh factor and why is it important?

The rhesus factor is an important characteristic of blood cells. It indicates whether the blood of two different people is compatible when mixed – such as the blood of a mother and her baby at birth. If they have different blood group characteristics, it may cause problems.

How do you determine Rh factor?

A blood test can provide you with your blood type and Rh factor. Antibody screen is another blood test that can show if an Rh-negative woman has developed antibodies to Rh-positive blood. An injection of Rh immunoglobulin (RhIg), a blood product that can prevent sensitization of an Rh-negative mother.