Baby size

All of these baby site have differnet estimates of the size of baby at any stage!! iT IS SO CONFUSING...I have no clue how big my baby is right bow because 3 weeks ago according to the sonogram he was like almost 3 lbs and if he is gaining 6 oz a week then he has to be at least 4 lbs? Weird..Oh well i will get another one next week and hopefully this time my doctor is doing it and not some US tech straight from school. That girl was such a jerk, FL HOSPITAL ORLANDO has a jerk of an US tech she was horrible.I wish I knew her name..