Just did a bit of googling
"I have an encouraging story: had my first ultrasound two weeks ago, thinking I was about 5 weeks and 4 days along (I had been charting faithfully, and was pretty sure about ovulation). The doctor could only see a yolk sac and said I was under five weeks. She seemed a *little* worried, but asked me to come back in two weeks. Well, I just got back from the doctor, and they found a fetus and saw a fluttering heart! I am measuring 6 weeks and four days (I had calculated 7 weeks and 4 days). So, it's entirely possible to be even a whole week off WITH CHARTING! And, apparently it was completely normal for them to see only a yolk sac when I was almost five weeks along. The thing that worried me most was that she said I was under 5 weeks, but I had gotten a positive pregnancy test 13 days earlier. Turns out my first positive (it was SOOOOO faint) was at FIVE dpo. Just goes to show you that no matter how well we think we're tracking things, there are always surprises. Take heart!"
https://www.babycenter.com/400_6-weeks-and-nothing-but-a-yolk-sac-seen-on-ultrasound_1005297_258.bc