Calling all my Ultrasound Techs!

On Friday I had an Ultrasound (38 weeks) to check for fetal growth. They’re concerned about my baby being too big for me if I carried her full term. The ultrasound tech was a bitch and wouldn’t answer any of my questions or give me any sort of info. I know that there’s things that they can’t tell you, but I’ve never in my life had an ultrasound tech act the way she did towards me. Anyways, as I was watching her take the measurements I kept seeing the EDD from each measurement she took though. One measurement was 34 weeks, one was 37 weeks, one was 39 weeks, and another was like 38 weeks I think?

How do they calculate the size of my baby from that? Do they just take an average?