Due date confusion

So I know it doesn't actually matter what day is my due date cause the baby will come when it's ready but it's nice to have a goal. 
So based on my period my due date would be September 17 but I'm irregular so I went for a dating scan and that put me at September 26. Then my 12 ultrasound yesterday put me at September 23. 
So one one should I go by?

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