How far along am I??

Briana
I recently found out by two urine tests and blood draw that I am pregnant! My last period started on September 4 and I'm think we conceived sept 15-19.... So it's been exactly a month since my last period but doctors aren't wanting me in til another month in order to be 8 weeks along.... How accurate is that? Can being a week or two weeks pregnant show up on a urine test? How early can doctors decipher my due date????