For milestones, do you count from birth or due date?

Amanda
I've read a couple places that when books and other resources talk about things related a baby's age that you are supposed to use their due date, not there birth date.  For example, my son will be 7 weeks tomorrow but was born at 37 weeks, so he should be treated like a 4 week old.  
Had anyone else read this or follow this guideline?