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. Having an Rh negative blood type is not an illness and usually does not affect your health. However, it can affect your pregnancy. Your pregnancy needs special care if you’re Rh negative and your baby is Rh positive (Rh incompatibility). A baby can inherit the Rh factor from either parent.