E.R.
My son's measurements- including his head circumference-- have been all over the place. He is currently in the 8th percentile for weight and the 60th for height. Am I worried? NO! He is very healthy and developmentally on track. Are you breastfeeding? Your daughter will not "fit" the charts they have at the ped's office, then, because they are made by FORMULA companies and apply to formula fed babies. There is a new chart, out from the WHO, that is more accurate for BF babies, but hardly any doctors use it.
Our family doctor does not even ever LOOK at the growth charts. What is normal for your baby is normal for HER. I really would not be concerned. She is growing fine otherwise, right? Gaining weight consistently? I would not be concerned at all. My son's head was small until he was about 6 months and then it grew all of a sudden. Nothing to be concerned about.
Just wondering--- is your hat size small? Your husband's? My husband has a smaller hat size, so a smaller head. Everyone's different, and I think my son just takes after his dad. Look at the adults around you. Nobody's head is the same size. :) Don't worry about it, mom.