Confused

So I've been on what I thought was my 8 weeks and 5 days. Then I have an ultrasound done that says 9 weeks 2 days. Should I believe what the computer says?? I know it sound kinda dumb but I'm just wondering if I should go by the computer or what I know. (Ps. The doctors said they wanted to keep to my due date that is why I'm so confused but the computer measured it)