How do you know how far along you are?

Stephanie • Married 1-11-2020 ❤️ MMC 4-2020 👼🏼 Praying for our rainbow 🌈

I just found out I was pregnant and I’m a little confused on how to know how far along I am. Based on my last period I’ll be 5 weeks on Monday. However, I know I ovulated late (on day 22) which would make me 4 weeks on Monday. Doctors will typically use your last period right? My first appointment with them isn’t until the 16th so I’m just curious.