I was unable to birth my first baby, who was 10lb2oz and had a 39cm head. After being induced via having waters broken and drip put up, I had a 15 hour labour with failure to progress, then an emerg c/s. After the c/s the dr told me that my uterus had thinned and was close to rupturing- due to obstruction of labour and a baby that was not going to fit through my pelvis. So in my case- the baby was too big to fit, but i do know of others who have birthed large babies.