How come my gestational age changed for my baby?

Ja

I had an ultrasound done and they said I’m a week more pregnant than I thought I was. How is that even possible if I know the exact start date of my last period ? I always log my period start and finish dates , hows it even possible ? I thought they went off of the start of your last period ? My due date moved from May 3rd to April 27th .

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