I had the worst back ever during pregancy no 2. I have always had a bad back though, and it got so bad this time that I couldn't walk. Started seeign my fabulous chiro at about 25 weeks and saw him twice a week for a few weeks and the pain eased. I had a 1 hour and 54 min labour and credit this totally to my chiro (he uses the gun thing, not manipulation). I also used a TENS machine on my back during labour. I was still sooo scared about labour thinking my back would play up, but infact I had NO pain in my back (highly unusual, all my period pain is in my back).
i did question having a cs because of my back, but was pretty much told that many women with bad backs give birth.
I would really consider a good chiro (who uses the gun) and maybe try the TENS machine!
Good luck xo