Yup, makes a difference. You're 2w pg when you O, which is usually 2w from LMP (if you believe the doctors). If you're 2w pg and 3w from LMP then you're still 2w pg, not 3w.

My hospital didn't listen about my irregular periods either and insist on dating me from LMP (so EDD of 31.01) even though all the early measurements confirm 03.02 - I'm amazed I O'd that early and refuse to give out an EDD as I'm sure I'll go over! My cycles were 32-55 days, usually about 6 weeks in length, so it's daft to just go from LMP and ignore the scan data, especially from early scans.