When does the water usually break?

I'm 32 weeks and recently moved back in with my parent's because I lost my job and I'm sleeping on their couch until my new bed gets here, and I'm paranoid about my water breaking when I'm sleeping and getting all over their couch... I don't want them to have to pay someone to shampoo the couch because of me... so I was wondering if the water breaks before or after labor contractions start? Or is it as sudden and spontaneous as the movies make it out to be? And is there any other "leakage" I should know about that might get on the couch? I sleep with a towel under me, but I know that won't completely protect the couch from my water breaking, and I keep having this nagging feeling that I'll be earlier than my due date.