You can’t donate blood while you’re pregnant because of the immune system changes in your bloodstream. Human Leukocyte Antigens (HLA)1 are part of this system, and when you donate blood, your HLA proteins go with it.
Half of a baby’s HLAs come from the sperm, and a pregnant person’s immune system makes antibodies to fight off the sperm HLA. These antibodies won’t hurt you or the baby, but if your blood gets transfused into someone with a different kind of HLA, it can cause a strong immune reaction that can hurt the recipient of your blood 2 .
Experts advise waiting until at least six weeks 3 after delivery to donate blood. Even then, the donation organization might still check your blood for antibodies. Feel free to ask your healthcare provider if you’re unsure what to do.