Can DD be off by a few weeks?

I feel like I'm way to big to be 30 weeks tomorrow. I sometimes wonder if my doc is wrong. My last period, I think was September 30. My due date is July 6th so conception probably the week of the tenth. My question is, do they go by last period even if your not sure, or ultrasound? Or both? My first one put me at about six weeks, then my next at 12 weeks last at 20 weeks. They never said anything about my due date changing and seems like everything is in track. I'm just wondering if I could actually be a few weeks further along and all this is wrong. They say I'm measuring on track.