Do doctors date pregnancy by 38 weeks or 40 weeks?

I got my hcg levels back. I was 6 weeks, the result was 9533. I asked “shouldn’t it be higher for 6 weeks”. She said, “technically you’re 4 weeks”. I then asked, “does this test go by 38 weeks or 40 weeks?” She skipped over the question grrr. I go for my ultrasound in a couple days and I’ll be 7+1 going by my last period. Will the US measure at 5+1 or 7+1? Hope this isn’t confusing......