Fiona, try this article Small Pelvis? Here's The Truth About Cephalopelvic Disproportion (CPD) as well.
Whilst in Labour my Ob said my pelvic opening was a bit small and obstructed dy divits in my pelvis, and was a bit worried, he explained that I would be preped for c/s just in case.
To his credit, he could have sent me for a c/s at that time (I asked him to give me one when this came up as i didnt want any complications) and he blantly refused, saying there is more risk from the c/s than at least trying a VB. 12 hours later DD was born through my small, divited pelvis with no problems but a tear.
3 hours later I walking around the hospital, showing of my baby!
It so can be done, the size is just an estimate, and it does stretch!

Good Luck with her!