Baby keeps measuring different

Hillary
So my first ultrasound baby measured 7 weeks. 
Then at my next ultrasound they measured him 10-4 days. Which was four days over what I was going by the first ultrasound. 
Then I had another ultrasound done and I was supposed to be like 14-4 days and I was measured 15 weeks. So a few days ahead. 
And then I had another ultrasound today and I was again measuring like two days more ahead. I am going off the amount of weeks that they tell me from each ultrasound , and every time he's bigger then what he should be. 
Is he just growing fast ? Are the ultrasound people just doing something wrong ? They measure his head and femur ... 
Idk I'm just confused lol 

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