Unknown Due Date?

Has anyone else had unknown due dates? With my first, my due date was a 4 week window and the last OB I had, I really liked and he picked one in the middle of the window. I don’t have periods without medication except maybe one time spotting a year due to my PCOS. I was on fertility drugs for the first miscarriage, my son and 2 miscarriages after him. Now pregnant again and got pregnant as we were starting a fertility cycle so we’re not sure how it happened unless the drugs stayed in my system. They’ve already said my due date is a 2 week window. My doctor says I’m measuring around 8-8.5 weeks and the ultrasound tech keeps telling me I’m only 6.5 weeks. I’m getting annoyed and confused because all my follow up doctor appointments are based on being 8 weeks. I don’t want to go and have to come back in 2 weeks to do another appointment when it’s a 2 hr drive one way.