Yes, Lulu is right true CPD (small outlet/big baby) is incredibly, incredibly rare. And "prolonged labour" can be a dodgy one too - i know a woman who was sectioned for failure to progress because she was "stuck" at 6cm for 3 hours! Her total labour up to the section was only 5 hours!
I'll second that. One of mine was considered failure to progress ("Stuck" at 5cms for 4 hours). Only they were *so* busy that they didn't even start discussion the options until 4 and a half hours later. They were in the middle of doing that when DH paged them because the head was out and the rest wasn't far behind. All I needed was time.