How does labor start? How do we measure labor stages? A walk through the research

Every human enters the world through a pregnancy that either ends with a vaginal delivery or a C-section. So every human on the planet has been through birth. technically speaking. Given that shared experience, it seems like birth, the how and the how long of it, would be a topic with a lot of relevance to many, many, if not all people. And, that maybe, we would understand what triggers labor and how to measure it's progress. But to date, there are lots of holes in our understandi...