aww don't panic!
this is my experience:
ds1- 9lb 1. horrid labour, very long, had epidural which slowed everything, ended up on my back, no idea when i was having a contraction, ventouse delivery, nasty tear
ds2- 9lb 5, born at home, stayed upright, no pain relief=- absolutely beautiful labour. no tears
ds3- 9lb 11, born at home in 45 mins! no tears again
I honestly believe that how easy (or not) your baby is to deliver has far less to do with their weight than it does with where you are, how you are coping, your position, the size of their head/shoulders etc!
I thought it was half a pound a week? am sure that's what sonographer told me.
I believe it's true that your body will never grow a baby too big for you to deliver. maybe your problems with your last were more to do with how the labour was managed than the size of the baby?