Short version: She wouldn't come out.
Longer version: I was fully dilated, drug-free, able to take any position I wanted, supported beautifully by my husband, doula, doctor, and nurses...and I pushed for 8 hours. The contractions became identical--she would come down so far, stop, then go back up when the contraction ended. They could see/feel her head, but her fontanelle was so small they couldn't guess her position. Thankfully she was never in distress, so no one pressured me or suggested I do anything not on my birth plan, and they let me push for 7 hours. Finally I and my husband opted for the c-section because no matter what I did, the pushes were all the same and no progress was being made. It felt to me that she was hitting the bones, and that they just weren't moving/opening. I was still strong, but I didn't think I should risk her health by insisting on a birth that just wasn't working out.
So here are the theories:
1) I think her head was tilted back as she came down, instead of being tucked properly. Her head was not molded at all, so this remains only a theory.
2) A doctor has since suggested (now that I'm pg again) that I may have a smaller-than-normal pelvic opening. But I realize that doctors want everything to be exactly like the textbooks, so I'm taking that with a grain of salt. He also thinks her head was probably tilted back, and that my pelvis was the reason why. Maybe he's right, but if we know that, can't we deal with it better the next time?
3) My doula has some info on birthing positions for women with different-shaped pelvises. I think I will also get a second opinion from a good midwife on my pelvic shape, and continue to plan for a vbac this time.
I no longer regret my c-sec because it was the best decision I could make at that time, and unlike so many women, I am fortunate that it was my decision, that I never felt pressured into it, or anything else. It was a mourning process, though, to get to this point. I will mourn again if I have another. But everything happens for a reason...I trust that. The main thing is to have healthy babies.