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? <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow app</a> 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 <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a>! 

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