I gave birth to my DD3 naturally posterior (face up). It does cause pretty full on back pain, but it can be done if it comes to that. You just need to keep active during the labour.

Labours with babies in awkward positions can often slow down & this is when things usually go hay wire. Any good m/w or ob will know that when this happens you need to get moving, but it is advice you need to know yourself, because often these labours end up as an emergency c/s as they see it as labour not progressing.

Get onto the spinning babies website, but also educate yourself & prepare yourself for what may be a more interesting birth There are a few birth stories on here that are posterior births, so go have a read of them as well, so you can see that it can be done

Good luck!