Measuring Ahead

So I know due dates are really just a guide, but I've told my OB and sonographers all along that my period was irregular and I received a very early positive test. My dating scan was 4 days out, but they kept the same day. Then my 12wk scan was 1wk out and again kept the same date. Now bub is almost 2wks ahead and they're concerned about the size. I've been monitoring my blood sugars to make sure it's not GD and theyre all fine. Now it's come time to tell work when I'll be finishing and I'm restricted by my due date. I have my GTT booked and also an extra scan to monitor the size of the baby but ultimately I'm not concerned as my gut tells me he's actually due 1-2wks earlier. Has anyone's ob changed their due date later on in the pregnancy?