GIANT BABY

I went in for a growth scan, and the baby is measuring 6lbs 2oz, and I am 34w2d. He was also measuring very large at my 26 week appointment. I wasn't getting a period when I got pregnant, and they normally base your due date off of LMP. He's been measuring 'big' since 12 weeks, and I've been negative for GD twice now. I think my due date is off, and I've thought so from the beginning. Have you ever had a due date changed? Or know someone who has? I know the ultra sound weight can be off, but it doesn't make sense to me that the ENIRE TIME he has been measuring 'big' and they haven't thought I'm further along.