I hope you get reassuring answers soon. I was in a similar situation as you when I was pregnant with my first kid. His head was always measuring on the small size (less than 5 percentile). Before he was born, I had such high anxiety over it, especially bc my spouse and I were in the French Polynesia during my first trimester (the South American zika was believed to have originated from the French Polynesia). When my son was born, his head measured in the 2 percentile. I was devastated but none of the doctors he has seen has ever brought up concerns since his head was proportionate to his facial features. For the first few months, I was hoping his head would catch up, but it never did. Turns out he just has a small head like his dad. The percentile measurement are literally millimeters. No one has ever mention his small head since it's proportionate to his face and body. He's just a regular toddler now, he has never had any developmental delays. I guess what I'm saying is, some kids just have a small head :)