FINALLY POSITIVE! CALCULATING WEEKS?

Mar

Hi everyone!!!! I finally got my positive test! I am over the moon ❤️ how do I calculate how many weeks I am? The first day of my last period was september 19. If I count 4 weeks from there, it says 4 weeks 6 days, but my glow <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a> app is telling me 4 weeks? Help!