I *think* the average hc is 34 cm, so anything smaller than that would be below averge and anything larger is obviously above average. I think it's just a mind over matter thing - I have had 34 cm up to 37 and there was absolutely no difference to me in how much it hurt, but no estimates were ever made on how big they would be. Maybe if I had of been told "oh this head is quite large" before they were born, then I think I would have gone into that birth with reservations about my ability to get them out. A lot of people have said 'OMG' to me when they knew how big my last baby's head was but it didn't make much difference to me at all.