I was recently in the exact same position, i prescribed a 5 day course of amoxycillin by the dentist who was aware of my pregnancy and assured me it was safe to take, pain and infection gone and my baby is fine :) I'm at 28 weeks. Ask a doc as opposed to a nurse, no offence to nurses but GPs have a better understanding of prescribing. Good luck x
You can tell your OB what the dentist prescribed and if he doesn't like it then he, himself, can prescribe something different. I was in the same position with my last pregnancy, but I don't remember what my ob prescribed at the time.
I had a UTI earlier in this pregnancy and they gave me something called Keflex, it cleared me up and I had no side effects. I'm deathly allergic to penicillin so I couldn't have that, but I would definitely go directly to OB as they are specifically trained in the area of prenatal care.