I'd go by your LMP, also, for the same reason as Calluna. The scans can be out, it happens. With my DD the early scan showed a due date 6 days later than my original - and our dates were solid since we did IVF. In my case, since I seem to have long gestations, it was handy to go with the scan EDD, since it was later. I would pick a date that suits you better and stick with it.