Rh factor is a protein found in blood. A person with Rh factor in blood is called Rh positive whereas one who does not carry this protein in the blood is called Rh negative. Marriage should be avoided between Rh-negative females & Rh-positive males. This can be fatal for the mother as well as the baby of such parents. (second pregnancy may result in erythroblastosis fetalis if necessary precautions are not taken).