How can I calculate me due date, I think my ob's date is off!

Im getting very frustrated, with my Ob, I have recieved 3 different due dates, and the differ from what my perinatologist has given me. When we get Ultrasounds taken, the date of gestation seems more accurate than what my Ob tells me. For example, He will tell me Im 27 wks along, 3 weeks later, he will say Oh your only 25 weeks along, im dumbfound it! Im most sure Im 36 weeks now, and Im very unhappy that I my Ob, seems to ignore my concerns and questions about my due date. I know My first date of my last period was 3-9-2011, I calculated that Dec 22-23, 2011, is close to my due date. Can anyone help me find some Peace of mind?!