Baby not measuring right?!

Hey ladies, I'm a little bit confused. I had my 21 week full ultrasound for baby boy. Everything was great tell measurements came, seems like his head is 19wks5days but has organs are all 21weeks. I asked the tech if that was okay and she told me she didn't know. Now I'm over here stressing, she didn't take a full body measurement of him either. Maybe I'm just freaking out but this is my second child and with my daughter I don't remember anything like this. Anyone else's baby measurements off by their week? 
Thank you ladies, I feel like this is the only place that will actually listen to my concern. ❤️ The support on this add is great!