ultrasound at 7 weeks and 1 day I was measuring 3 days ahead

My last period was june 18 , 2012 when I got my ultrasound at 7 weeks and 1 day I was measuring 3 days ahead. With a due date of march 22, 2013 with an conception date of june 29th. But they kept my due date at march 25th 2013 , with an conception date of july 2nd does my conception date changes because of me measuring ahead . Or does everything stays the same ? I really need to know ! Someone please help!