I'm lurking from 1st tri, but my last baby was posterior and I was so scared. I was going for a drug free birth and i was nervous it was going to be longer and more painful.
He stayed posterior all the way up until I start pushing. When she had checked me 45 minutes before he was still posterior, then when i started pushing she said he had turned. He was out in less then 10 minutes.
I know it sounds hippy-ish, but I think most often babies and our bodies know what they're doing. But I get that it is hard to not be trying something.
Either way, try not to worry. :) GL!