My docs go by the earliest ultrasound. No matter what you measure in future, its not as accurate as the earliest ultrasound you had. Some babies are tall some short some chunkers some skinny, that's why they never change my dates. My ultrasound tech always says my due date should be this or that but my doc ignores them and sticks with the date given on my first ultrasound. I'd definitely talk to your OB about this!
