I measured one week ahead at our NT scan from memory (and it may even have been at a nine week scan I had as well) with my current pregnancy. We conceived using Clomid which triggered my ovulation and I am sure my dates weren't a week out (it's pretty easy to tell when you are ovulating on Clomid). Doctor let me take my pick for my estimated due date (what a scientific way to do things!!!!). I am sure the baby couldn't care less which date I picked.