Unbearably breathless pain, you don't get to clearly point out exactly which spot hurts like a toothache, but a pain that's in the range of a slice + hard enough that I wanted to throw up all the time, hard enough that my whole body convulsed when I opened the last three fingers, and worst of all, usually it's the kind of pain that lasts a dozen hours or so.
I gave birth entirely to small abdominal pains and felt no significant back pain.
The best description I can find for this pain is - a small curvy abdominal explosion of pain.
First of all, about the "curvilinear". Birth isn't a constant pain, it's a paroxysm of pain, and paroxysms of pain only come with contractions. The contractions come about once every 3 minutes after labor begins, and each contraction lasts about 1-2 minutes.
That means that each pain is only about that 1~2 minutes, and after the pain is over it goes into a completely painless 3-minute interval until the next contraction comes. And the contraction pain doesn't peak all at once, it's a gradual, curvilinear increase that slowly diminishes as it reaches its peak.
The second thing is the "explosion of pain". Every time a contraction comes, it feels like someone is holding a pump and starting to inflate their abdomen, every deep breath, it is inflated once, every inflation is accompanied by the pain of the explosion of the abdomen, until the abdomen reaches the apex of the explosion that is about to tear the heart and lungs,
You will start to feel the "deflating" with every breath. Then wait for the next contraction pain. I've never had menstrual cramps, so I don't know if this is more like what menstrual cramps feel like, but constant pain like this for 10 hours or so really isn't pain that's easily tolerated.