Why can't you give blood if you've had a postpartum transfusion?

Possible Ab reaction. If you have a postpartum transfusion, you might get an antigen that you can develop antibodies to, especially if you are rh negative. This would complicate things like future pregnancies and transfusions to you, if you have another. So children and childbearing women are given type o, rh neg. Blood. I don't think it would affect your ability to give blood though.