Question to you mommies out there ???

I had an ultrasound yesterday where my little boy should have been 17 weeks 6 days and he was messuaring 18 weeks and 3 days is it normal for him to be 4 days ahead or bigger then what he should have been ??? I'm a little worried about it his heart was normal brain was normal all blood flow was normal my ultrasound tech. That was in the room with me said he was perfect and healthy just showing bigger then what he should have been so I am a little worried about my boy help please all comments are welcome