Has your fundal height been off at all? And it also depends on the U/S tech, some don't do the most accurate measurements, either, which can throw it off, and if your doctor isn't concerned I wouldn't be either. Babies gain about 1/2lb or more a week, so if your little one gains that and is 4lbs, he/she will be about 6lbs at birth, which is a very normal average weight, so don't stress!