Pushed back due date!

Allie
Just went to the doctor and during the ultra sound moved my due date from Sept. 9th to the 16th. A couple of things:
I'm pretty sure that I know the day we conceived and this new due date puts us 10days off. 
The ultrasound was REALLY blurry. The doctor even commented on it. 
I was really worried about the baby not growing, but doctor says the heartbeat is strong at 157. Has anyone experienced a blurry mismeasurrd ultrasound? Maybe I just don't want to add another week to my pregnancy...