Help!! Age of baby?

Ok so I found out I was pregnant 12/26 but I had NO CLUE when my last period started. So when I went to the dr, they did an u/s and the age of the baby said 8 weeks 6 days. Well my dr I saw that time (there's a few in the office) said we had to add two weeks to that if we go by the lmp date to get the gestational age. So the dr I saw today said that the date on the u/s accounts for that and is the gestational age already! I'm so confused! This means first of all that I did nipt testing at 9 weeks and it's only accurate after 10. Also 2 weeks may not be a big deal now, but at the end of like to know if I'm 38 weeks or 40... 40 or 42?!? I know it's never accurate and you can always give or take 2 weeks but still.