how to know how far along I Am?

Tapp

so I know you count from LMP which for me was July 8 but my cycles have been a bit irregular and longer. I ovulated on July 26. so if I go by LMP then I'm 5 weeks and 3 days but if I go by ovulation then I'm 4 weeks 6 days (I think). I know it's not really a difference but i would like to know which is more likely. anyone have experience like this? any advice on what the doctor would say? (I don't go till Sept 18). thanks!