Measuring 6 days behind on dating ultrasound

Wondering if anyone else has had this happen to them. I had my first ultrasound at 8w3d and was measuring 6 days behind at 7w 4d. I'm not worried about this because I know it is common but I don't want my doctor to change my due date because I am confident in my dates. My lmp was Aug 10th (my cycle is very regular) and I used opks so I know I ovulated on Aug 24th.

A lot of women measuring behind have irregular cycles or could have ovulated late. Has anyone used opks and had this happen to them?