How do you know how many weeks??

lainie
Hi! I am preggo and my <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> says 4 weeks today ... But I think I only conceived 3 weeks ago (if the fertile window was accurate!) how do you count before you actually go to the dr?? I don't go for bloodwork until August 31.