6w vs. 3&1/2w Confused!😳

Shane
According to my LMP I'm 6 weeks today. 
According to when I ovulated and had sex I would be about 3 and a half weeks today. 
I'm so confused as to which is more accurate.
My first appointment isn't till I'm 8 weeks anyway and if everything goes smooth and I get to that appointment wouldn't I measure smaller then? 
Pleass Any insight anyone?