Ultrasound Measurements VS Due Date

Sarah♥
They say at my 20 week ultrasound that the baby (We're having a Boy) is measuring 18 weeks (2 weeks behind), so do I have to change my due date now or keep it the same as my doctor gave me. Then at my next ultrasound if they confirm the same thing, than change it? I'm confused now on when my due date is, Help?!