Question about measuring ahead

kimjellie

We had our NT scan today, and while I should be 12w2d (based my measurements at my 7 week appointment and the due date by OB-GYN gave me at that time), I measured at 12w6d.  Does that every change back again, as in, should I still think of my due date as December 4 for now and see what happens, or once you measure ahead do you pretty much always stay there, such that my due date is probably now Nov.30?  I think the only reason I am anxious to know is it means I am four days closer to being in the second trimester (when I am hoping the m/s will soon cease!)