Confused!

Hey ladies, I really need your input. I am pregnant, and have always calculated my dates by going off the first day of my last period, which would make me 11+1 today. I am considered high risk and go to a speciality clinic, which means I'm fortunate to have lots of u/s. When I have gone for my appointments, the baby measures four days ahead of where it should be. So, the question is, do I count myself as 11+1 or start going from the other date, making me 11+5? All of this stuff can get confusing lol, so thanks in advance!