Pearl, I don't have any references in front of me but I am a childbirth teacher and a LLLL and from all of my reading on Rh factor the only problem comes from someone who is Rh- getting any Rh+ blood, they will produce antibodies that would be dangerous for the unborn baby during pregnancy (I don't know what applies to men). I know that the body's defense mechanism can cause problems, but only in Rh- people. That is why when blood is needed and the blood type and Rh factor is not known they use O- (O being the universal donor, and Rh- to prevent any Rh antibodies from being produced). So if any of her Rh- negative blood mixes with her baby's Rh+ (very doubtful from breastfeeding), it shouldn't bother the baby. The risk comes if she gets any of her baby's blood in her blood stream (which can happen when the placenta is delivered). Hope this helps. Felicia LLLL and MOM of 2