E.M.
If your doctor is basing how far along you are on your last period, that can definitely be off! Those little charts they use make the assumption that everyone has the same, 28-day cycle, and that everyone ovulates on a certain day in that cycle, and that everyone conceives the day they ovulate. That is a lot of assumptions! You are a person, not an egg-dropping robot, your cycle could easily be different from the one on the chart. For example, with my second pregnancy conception happened 10 days LATER than the chart said it "should."
Try to relax, and best wishes for a healthy pregnancy!!