I have had a similar thing, doctor gave me date of 14th October which was confirmed at 6 week scan, then at nuchal scan it was 4 days bigger than that date, and at 19 week scan its now 8 days bigger......I am panicking a bit too as I dont want a big baby! I am very careful with my diet so i know its not overeating thats doing it. The scanner lady said it might be because my cycle was shorter than the average, but I still dont think that would contribute to that much difference. I am hoping my doctor will adjust my due date when I see him on Friday! Sorry i am not much help, but am interested to see if anyone else comes up with any thoughts for you :0)