I thought i was 8w3d at my scan last week but was told i was 7w1d. (due to egg dropping late!!!)
Very disappointing....but everything was fine, he reassurred me.
But just wondering as well.....which week do i count?
I would go by the u/s date (the measurement of the baby). That's the one the OB will use anyway so you might as well use it.

The whole counting from the LMP only works properly if you have a "regular" 28 day cycle. When I showed my OB my FF chart he said my new dates made sense with the length of my cycle and when I O-ed etc.