Rh factors
A group of antigens that may or may not be present in red blood cells, forming the basis of the rhesus blood group system. Most people (75%) have the rhesus factor and are Rh-positive. People who lack the factor are Rh-negative. Incompatibility between Rh-positive and Rh-negative blood is an important cause of blood reactions in the growing baby. If the expectant mother is Rh-negative and the father is not, she will need to receive an injection to prevent the development of antibodies that could be hostile to the baby.