I'm confused with the due dates

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Hi mama-to-be

I'm so confused with the due date.

Ive tested on Tuesday using clear blue digital with a positive 2-3weeks. My last period was 16th Dec so how many weeks am I?

Cos I get 22nd Sep and 3rd Oct, but I dnt understand now it works.

I'm getting really impatient to get my 1st scan on Thursday and worrying I can't scan anything.