I am not sure of the medical facts, but in my experience with Olivia and Charlie I had the worst back ache with Olivia (posteria baby) and no backache with Charlie (anterior baby). So I guess there could be something in it?

With Olivia the backache started at about 37 weeks and she finally made her way into the world at 41 weeks.....so I had a month of shocking back pain.....then whilst in actual labour with her my labour pains where all around the back of my hips. (With Charlie it was all round the front, but his was a quick labour, so it doesn't stick in my mind as being as painful!)

HTH!