the same thing has happened to me, the dating scan has put me back a week. It seems to happen to most people.
but i am still going by my LMP. just cos it sounds better and I'll wait and see what they say at my 12 week scan.
with the scan i'm pretty sure in the information they give you it says you can have it anytime between 11 and 13 weeks, so i just booked somewhere in between.
dont stress about it too much, 2 weeks wont make a huge differance, maybe see what your doctor says.