Due Date Change


I've had one ultrasound, when I was 7wks as a dating scan. Due July 25th, however that was one week behind where I thought I was. I go for another ultrasound at 20 wks, 3 days. I measured my fundal height, and according to that I'm measuring 20 weeks, even though I'm only 18 weeks 3 days. Has anyone had their due date changed??