Which due date I should believe? Please read my description before you vote thanks

Jocelyn • First time mom and super excited!!
Am wondering which due date I should follow? Glow app says I'm 13 weeks today (due date is October 2) but my doctor told me it's October 7. 
I know my doctor based them on my ultrasound...Two out of 4 of my ultrasounds are matching my glow app

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