What date do I go with and us it accurate

Hi ladies just wondering if any of you have been given different due dates up until your 12 week dating scan , I had a scan at 5 weeks was given a due date then another scan at 7 weeks with a different due date . I finally had a 12 week scan and they pushed my back backwards so I gained 3 days back, is that a accurate measurement or should I stick to my very first date given :)