I went through 12 hours of natural labour with zero pain meds with my baby in the posterior position. ("..this labor is quite a bit more painful as the baby essentially “rides down the spine”.  In this position, the hardest part of the baby’s head is pressing against the pelvis bone.  The pressure that results from this pushing causes significant back pain for the mother.") More explanation here.

I could feel my pelvis slowly being split open with every contraction and I didn't progress for a long time. The back pain was excruciating. I had to go somewhere else in my head, but kudos to my husband for being there with me through every second of it. I found out later when they took my baby's measurements that her head was in the 99th percentile for size. I'm a petite person and my husband blessed our baby with his big head genes. :l

Needless to say, I decided not to have any more kids. One labour experience was more than enough for me. :')