If you were charting and know when you O'd then be confident about your bubs EDD. If you have longer cycles then your OB may just take a week off the start of your cycle to make you "normal" as everyone wants to know the date of your LMP and it's easier to do that than to tell computers your cycle is 35 days (well that's what they did with me anyway )

I've had EDD's moved about, brought forward and then moved back at later scans. It's a bit annoying especially if you need to plan DH's leave for the birth in advance.

They will give you a more solid date at the 17 week u/s.