K.F.
Could be any number of things. Are you absolutely certain that the conceivement date and due date are right? It could be that you're just a month further along than you thought you were. The possiblity of twins is definately something to look into. I have heard of people not knowing they were carrying twins until later because the babies heartbeats were so close together that it sounded like one. As far as ultrasounds and them saying how much the baby weighs....I had four ultrasounds done in my last three months of pregnancy. The last one at 38 or 39 weeks, and my little girl was not supposed to weigh any more than upper 7 lbs. She came out weighing 8 lbs 11 oz!
I wish you the best of luck, and let us know what you find out!