Anna, if you're a small-ish woman, then you definitely need to have a look at the links I posted earlier. He's a male midwife who worked with Tijuana (Mexican women) who are very very tiny. However, they delivered healthy average sized babies at home. One of the girls on his site was only 12!!!
Even head circumference is not much to go by. Baby's heads are soft. Your pelvis is soft. There is no way to tell how much the head will be able to be molded during birth and how much your pelvis will be able to stretch.
This article on Belly Belly is also really really interesting: https://www.bellybelly.com.au/birth/...nown-and-loved.

Seeing that you have a fair idea of when you ovulated, there is probably a considerate accuracy in your dates, rather than the u/s dates. I would definitely go by your dates rather than by theirs.

I had long irregular cycles (45 days+) but I also knew when I conceived. Originally, with my GP, she thought I was 2 weeks further along and wouldn't believe me. When I went to see my MW, I mentioned the charting/long cylces. She just asked me: So, what due date do you want me to write down? I loved her for this. It is so great if they believe your instincts. I did end up having a dating u/s at 6 weeks and that confirmed my estimate to the day.

All the best and try not to worry.
Saša