I am also 20 weeks this week! Due Sept. 30th. We must be very close to each other. Anyway, I'm seeing midwives at Methodist. They only did a vaginal exam at the beginning because I had some symptoms of a bacterial infection (which I did in fact have), but they would have held off even on that because I had had a pap smear within the year. They said they won't do one again until 36 weeks now. Seems to be what happened with me my last pregnancy too. Anyway, no one wants to be up in stirrups any more than necessary, right? :)
Best of luck to you!!!