I stumbled on this thread after a Google search looking for answers about back labour. Don't know that I have much to add but I did find the other comments interesting, and thought I'd share my (and my daughter's) experience just as a point of reference for others.
I had three children - the first two were boys, and I had no back labour at all with either of them. The first was 7 lbs 2 oz, the second 9 lbs 2 oz.
My only experience with back labour was with my last, my girl , who was 8 lbs 7 oz and "sunny-side up" during an ultra-sound done earlier on the day of her birth. I've always heard (and my experience obviously bore it out) that back labour was the result of a baby facing the front. After 6 hours I was only dilated to 3 cms, and was advised to go stand in the shower and let the hot water run on my back. This helped, but what helped more was that in leaning against my husband, she suddenly turned and her head was then pressed firmly against my cervix. The back labour stopped immediately and I went from 3 cms to 10 in less than 30 minutes! She was born before the Dr could get back to the hospital!
Ten days ago my baby girl had her first baby - and she had horrible back labour the entire time. But her labour went very fast, in spite of it. She was fully dilated and delivered in less than 6 hours - so not really indicative of a "sunny-side up" position, although the ultra-sound at 20 weeks showed the baby facing forward. That's early, though, for positioning - and I'm sure the midwife had indicated she'd turned since then. I didn't think to ask during the delivery - things were pretty intense. One thing I did notice, though, was a mention in these comments about an anterior placenta - which is not common - but something my daughter had. I don't know if I had anterior placenta with her. But maybe that's a factor? Also, my granddaughter was a relatively big baby - as was her Momma - 8 lbs 10 oz. Anyway, make of it what you will - I have no explanation but there seem to be some common factors there. In the end, what helped my daughter the most was placing my fist into the one really sore spot in her back, and she leaning into that through the contraction. The discomfort increased during active labour - but she didn't seem to feel it between contractions - and it definitely let up by the time she was in transition and pushing, which was a relief.
Anyway, the important part in all of this is our beautiful little ones being born safe and sound and lovely. Congrats to all - it is such a privilege to bear a child . . . if I still could, I would. Now I just get to love my daughter and be enormously proud of her for giving birth to my beautiful little granddaughter - as the life-cycle continues . . .