Due date

Elina

I have a couple apps that I use and 2 say 4 weeks 1 day and one says 5 weeks 1 day. Counting from the first day of my last would be 5 so why are the others saying 4? Glow is one that says 4. All I know is I am soooooo exhausted because on top of the pregnancy I have 4 kids and work full time