In Rh negative mother, who has delivered a Rh positive baby, prophylactic anti-D is indicated:

Indirect coombs test is done in mother- if it is positive means that the mother has made the D antibody. Direct coombs test is done in baby - if it is positive it indicates antibodies to the Rh antigen is already made by the mother and are transferred to the baby. Giving an Anti_d to the mother is of no use anymore So in these conditions anti D antibody cannot be given.