Confused

Hello, I’m after a bit of help as I’ve confused myself...!

I recently found out I’m pregnant but I don’t know how best to calculate how many weeks I am.

The first day of my last period was 25th October but I was tracking ovulation and got a positive opk on 8th November. So do I go by my last period or by my opk result? It doesn’t make a huge difference in weeks but I’d like to know if poss.

I have an appointment at the doctors on 20th December so I will ask him but wondered if you ladies could help in the meantime?

Thank in advance! 😊 x