Due date confusion

So originally my due date was March 22 and then a few weeks ago my ultrasound tech told me that I’m measuring 3 or 4 days ahead of that and my due date would be March 18 but then I went to my doctor checkup today and she said it stays the same so I’m confused which I should go by lol I know it’s only a few days but I feel like a few days is still a difference lol. Any suggestions on what I should go off of?