Hi Claire ... I had somewhat of the same experience. I had my first ultrasound @ 16 weeks and the doctor wanted to bump me up two weeks. The doctor then scheduled another ultrasound to check size (which I got billed for!) and again, said my baby was measuring big. I was extremely frustrated b/c my family has a history of larger babies (8.5lbs-10lbs).
Going on the "new" date, this means that I would've gotten pregnant and still had a period and been on birth control. (I was pregnant 3 weeks after removing my patch) ... I knew exactly the date I had gotten pregnant and the original due date was correct.
Yet, the doctor insisted and when the new date came and went, I was pressured to induce. I finally went in but felt really defeated, knowing my son wasn't late yet. I was in the hospital @ 9 am and it took until midnight to be fully dialated. And even then I had to push for 3 hours b/c my son was no where near arrival. I was so afraid they would move to a C-section and I was in so much pain.
I am still convinced I shouldn't have delivered at the new date ... so stick to your guns. You know your body and would never put your baby at risk (doctors tend to scare you when you don't readily agree to inducement).
Perhaps you are having a bigger baby???
ps ... my Big baby was only 8.5 lbs ... and he Still measures in the 90%, meaning he is tall/etc for his age.