Okay ladies I need a little help understanding the week of pregnancy....

this app says I’m 3 weeks pregnant based on my missed period. (I was supposed to start Feb 5)

However every over <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">pregnancy app</a> says I’m 5 weeks pregnant. Can anyone help me understand which one is true and why they are two weeks different

I do know I’m pregnant I had a blood test done I just wanna understand more since I don’t have an ob appt for 3 more weeks