Is the dr right ??

I had my first ultrasound as I was 6 weeks supposibly. The Dr said the same thing , from my last period date that's how many weeks I was it didn't change when he did the ultrasound , would he be correct this early ? Or is it possible I'm 2-4 weeks more pregnant than he thinks? I have another ultrasound in a week and I'll be 9 weeks supposibly then. I'm just trying to make sure I got pregnant at the time they said and not earlier.... Please help!