I also declined the rhogam prenatally. My understanding is that the risk with sensitization is not so much for the current pregnancy being carried but for any future pregnancies. You would have 72 hours to get the shot in the event of a trauma, same as after birth. I would assume you'd have some sort of symptom like pain or spotting to accompany a bad enough trauma to cause blood mingling, but I am not sure. That's just how I'd think of it for myself. If you are really worried, don't worry about bothering the midwives. That's what they are there for, and maybe their reassurance will be what you need to hear in the end. I'm sure they'd rather spend 5 minutes on the phone with you than know you are fretting.
Oh, and there's the chance the baby isn't + anyway - my two heterozygous positive parents conceived negative ol' me! You are -- for the protein, but if your husband is a heterozygous positive (+- instead of ++), he may have donated his -.
Finally, if the baby is +, you can also get your titers checked for the antibody after delivery, which might help you feel at ease for a future pregnancy.