I think those things can be wrong very much. On the day i had my son, they did an u/s estimating him.at around 6 lbs...at 40 weeks. He was born later weighing 9lbs 1 oz! And my last baby was only 6 lbs. I have 11, this is my 12, and it seems that their weight is as individual as they are. I always measured behind in my last pregnancy. Try not to worry, most of the time those things are off.