Small head

Since I have been 20 weeks my baby’s head has been measuring two to three weeks behind. I have been going to a growth specialist to try and figure out why his head is smaller and what can cause it. I go every week and get an ultrasound. They check his organs, blood flow and umbilical cord and every week they say the same thing “he looks perfectly healthy, just has a small head” which makes me wonder why I have to keep coming. Or why I can’t come every two weeks. The appointments are an hour away, cost $50 each time, and I have to bring my 2 year old after working all days. I’m glad they’re being better safe than sorry but it’s just a lot for this tired mama. Has anyone else experienced this. I really just believe he’s okay and has a smaller noggin.