C.M.
I too am RH and I had 2 pregnancy. For my first I received the shot and for my second did nothing. My mother is a midwife and I know: When your negative and your baby is positive your body will reject the baby most of the time (this situation is common). If your type is battling the baby's type you end up mostly in miscarriage, but if for some reason you still end up with conceiving the child the problems that occur are mostly jaundice and perhaps other complications (to baby only). This mostly has to do with antibodies. That's why at 27 weeks they normally make the decision of wither or not you want the shot because that's when they catch if the antibodies are there or not. You can research this on google.
You choose. So if your babies blood type is a ok with yours no need for the shot. Good luck!!