Hello. I am a preggo postdoc. I have been nothing but well-supported so far. Caveat: I'm in a multiple-year postdoc.
One thing I didn't realize was that pregnancy could be so physically difficult. I'm not sure how I would have managed internship (or an intense clinical postdoc) while pregnant. I have been fairly sick and exhausted. I think you can more carefully choose your postdoc than your internship (because of the Match), and I'm glad I waited until postdoc, though I also had other personal reasons for waiting. It's heartening to hear that many sites are accommodating to pregnancy.
I personally have known two pregnant interns at different sites. One was due the fall after internship ended. She arranged to start postdoc late. Though there are dr. appts and such, she didn't lose so much time that our sick leave didn't cover it. Another one gave birth during internship. She took leave during internship and made up the time at the end.