How does my doctor know when I'm getting ready to go in labor?

I'm only 37 weeks, but the doctor thinks I will go in labor this week. They have been talking about inducing me because of low amniotic fluid, but they are waiting because she thinks I will go into labor on my own very soon. I'm not dilated but 1cm, so I don't see how. During my stress test they are picking up contractions every 5 mins, but they aren't painful so I'm guessing they're braxton hicks. She has been engaged for over a month now, so I don't if maybe that has something do with her decision. Also the ultrasound showed that she is practice breathing, and over 6lbs. I don't feel like I'm going to have her soon, what would make the doctor think this?