Rh factor is the protein present in blood and blood groups are termed positive or negative on the basis of the Rh factor. If the mother is Rh-negative and the father are Rh-positive, the mother may get aborted because of Rh-incompatibility.
When Rh antigen introduced into the circulation of Rh negative person, it stimulates the production of antibody in the form of agglutinin. Two forms of agglutinins are formed i.e Albumin agglutinin and saline agglutinin.
In Rh sensitised mother, antibodies can be detected at 34th week of pregnancy. Usually the Rh-compatibility test is conducted twice during pregnancy. If the mother is sensitised during last pregnancy, antibodies are found in the early stage of next pregnancy.
Blood group of only 15 to 17% are Rh negative. The appearance of antibodies for first time in the later stage of pregnancy indicates, the fetus is stimulating maternal antibody production.