Confused about whether to have anti-d injection?

I have been told that I am Rhesus Negative and have been advised to have the single Anti-D injection at 28 weeks. My Husband's mother is convinced that he is also Rhesus negative as it says on his hospital notes from birth that he is A-. I have read two negatives parents cannot make a positive baby, but I am nervous about refusing the injection out right based on this. Can anyone offer re-assurance / advice that this injection is okay to have, even if baby is infact negative too...