No, my early labours have been caused by kidney problems and infections every time. I had to take medication to stop labour in every pregnancy of mine, even my first (from 19 weeks already!).
I do think if I had stronger contractions that's closer together in my 4th pregnancy, that it would've ruptured the uterus. But I didn't feel scared so much during my third. I'm probably wrong and should've been scared, but somehow it just wasn't that big a deal at that point?!
But with my 4th I just knew that it could all go wrong. It started at 34 weeks already when I went into early labour and spent a night in hospital to stop it.
Just to clear up. Even after my 3rd c-section and the fact that it thinned, my dr still told me afterwards that if I wait long enough, I can go ahead and have a fourth.
I believe that as long as your doctor knows all the risks of your individual case, you and baby will be fine. It doesn't stop us from worrying, but it's reassuring if you have a dr that's good and that will keep an eye on you. They know when to keep a close eye on you. Try not to worry too much, at least not during the first 2 trimesters. And just keep an eye on what you feel. Rather go in too many times to be checked out.
Some of the symptoms I experienced or were told out to look out for was pain the scar, especially when the pain doesn't go away. I had pain in the scar all throughout my pregnancies, but it would go away for a day, then come back, then go away. When it doesn't go away but worsenes in intensity, that's when you need to worry. Also, it's a burning feeling when it starts to tear, not a constant dull ache. A friend of mine whose uterus also started to tear at 36 weeks, said she could barely walk into the dr's office because her scar was so sore. Blood in the urine (making it dark/brownish) is also a sign.
Good luck!! I hope you'll be pregnant soon, and that it will all go smoothly.