It seems that every ultrasound I have had in second trimester has measured my little guy differently. Sometimes he is right on according to dates, other times way ahead, etc. That is why they only use the early ultrasounds for dating purposes and don't change your due date based on later ultrasounds. I highly doubt there is any problem with your baby, they all grow at their own speed. Congrats, your getting close...!!
You may just have a smaller baby. I don't think it is harmful to the baby, as ocean said, it may just have a lower birth weight. If your doctor is not overly concerned, then I would try to relax.
Did they give you a percentile? My baby was in the 25 percentile at 36 weeks, she was taken out at 37wks1day and is perfectly fine (was small at birth 5lbs12.5oz) but seems normal otherwise.