Our son was born on November 21st, but due on December 22nd. So should we expect him to hit a 3 month milestone when he is actually 4 months old?
I decided to throw this answer in here for completeness, to be a little more general.
My first was 4 weeks early, and we calculated based on his actual birthday. This was based on advice from our pediatrician. (and he did, and he caught up on height/weight by 6 months)
However, 4 weeks is not that early (given that in the US, 3 weeks early is NOT considered a early, but within the norm).
I know that most many early babies are expected to hit milestones according to their due date, especially if they are 6 or more weeks early.
In the general case I would definitely ask your pediatrician at the check ups how s/he would prefer to calculate, and inquire why.
Mine was a month early too, and her pediatrician seems to expect her to hit things based on her birthdate, but said not to worry if she was late. For instance, she wasn't social smiling at 6 weeks, but was at 10 weeks. My sleep book (Weissbluth) says to expect babies to develop sleepwise based on due date, not birthdate.