My doc changed my due date?

So before I even went to the doc I had it counted out and my due date was august 26 and when I had my first appt they never said anything except that my estimate was perfect but when I went to my second appt they said they was changing it to sep 6 but when I got my anatomy us they said she was practicing her breathing which she should be 21 weeks doing that and that she was already 12oz but at the time where they had changed my date I should have been 20 weeks exact I honestly feel like the first due date was more accurate which should I follow tho like has anyone else had this prob if so which date did u stick with???