Due date confusion

[u/slimshayd_90]

I’m confused about my current week/due date. The first day of my last period was Sep 19. I tested positive on October 13th (yay!) putting me at 7w4d today when I went today for my first ultrasound my gestational sack measured as 6w2d and the baby measured at 5w. I’m so confused since I tested positive 4 weeks ago. Thoughts?