Last marked period or ovulation day?🤔

So my lmp was August 12 but thought opk and temping I know I ovulated September 1, which is later than “average”. When estimating my due date what date should I use?

I went to the dr when I should have been 4 weeks according to my lmp and hgc levels were “very early”

I’m thinking it’s because I ovulated late and was actually only 3 weeks. I did go back 9 days later and my hgc is riding appropriately and in the 8,000s, just wondering what date to base my estimated due date off of!!