Why do they count from your first missed period ?

Why do most doctors and the apps go from your first missed period.?!

Just curious. I have been TTC for four years. Just got my first positive ever in that time frame. My last period was April 6th. I ovulated around April 20th.

Got a BFP on May 14th. Confirmed with digital. Went for blood work yesterday. HCG was 803. Does that seem reasonable ? I know the second blood test is the one to show the doubling of numbers but as of now I’m not scheduled for another blood test.

I have one daughter who is now 9 and was very much a surprise and I hide my pregnancy for a while. Now after trying for so long thinking it would never be me again, I just feel like it isn’t real and am having a hard tome not feeling completely stressed !!! 😩

Any advice for me ? Lol

Sorry as I haven’t told anyone other than my hubby yet, I’m looking for people to talk to!