Can your due date change in 3rd trimester?

So I've seen a few other posts where the ladies have said that they received an ultrasound 30 weeks or later and because the baby is measuring ahead, their due dates have changed. I had an ultrasound at 9 and 19 weeks and my doctor told me that since I am low risk and haven't had any complications, I probably won't be seeing my baby again until he's born! We listen to the heartbeat and they measure my belly each during appointment but what if he is measuring ahead and may come sooner than my due date? How would I know without one more ultrasound? Do I just have to be ready because you never know when I will go into labor? Obviously those are questions I will have to ask my doctor but I was just curious if anyone else has been in the same position as me?