different due dates each u/s?

TX-Bride

I start by saying that I was not charting, so my due date calculation at my first visit with the dr was based on the last period. A week later I had the dating u/s and it was two days ahead, so the dr said, that's because you ovulated earlier, but we keep the original due date based on the period, becuase it's only two days difference. Yesterday I was spotting, so the dr decided to do another u/s and on the printout he gave me, the due date is pushed two days ahead of last u/s (4 days earlier than the period due date). Has anyone encountered this? DH made a joke that baby is in a hurry to come out!! But how reliable are those dates? Is my dr not measuring it right? is something abnormal going on? I am a little nervous, I am 37 and had a previous m/c.