I have been lucky my ob goes with the date given at the 12 week scan and that's it. I always measure behind after that but the scans aren't as accurate later on, so I don't know why the EDD would be adjusted continually. I do wonder if I am a week earlier as the scans push me back a week later and then I measure inline with my LMP but it gives me an extra week up my sleeve before any intervention not that I have needed it with two pre term labours.