Actually, I don't think the exact mechanism by labor is begun is known! I know, there are theories, but I don't think they're really sure.
I do know that one common finding for a spontaneous PTL is an infection, either a UTI or chorioamnionitis. I am also aware that crack cocaine can cause the uterus to contract strongly enough and for a long enough time that the placenta can abrupt (or begin to). But there are other things that can trigger PTL (multiple gestation), or cause the woman to need be delivered early (pregnancy-induced hypertension).
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