Pediatrician is concerned about growth of head

Vet-Tech

I went to the pedi today for a recheck of length, weight & head circumference because they were concerned my DD's head is growing faster than the rest of her.  He doesn't seem genuinely worried there's something wrong but he wants us to get an ultrasound of her brain before her fontanelle closes.  I scheduled it for Wednesday but the ultrasound tech said they probably won't be able to see anything definitive because of her age.  The doc wanted to start with this before radiating her with an MRI.  Has anyone had experience with this at all?  I'm trying not to worry since she's meeting all of her milestones & there's no developmental indication that there's a problem.