With my first I started mild contractions the day before. I went to the doctor the following day and to the hospital later that evening. So with that I did have some warning. The next I knew but had only a couple hours.
Nope. Some people can be dilated for weeks before delivery. Some people don't start until labor has begun. Like the previous poster said, you could go to bed feeling perfectly normal and wake up in full on labor a few hours later.
Just happens. Two of my kids I went to bed felt normal, woke up in labor hours later. :)