Ultrasounds were sure my 1st son's head was in the 99th percentile, huge. I was induced because of his big head. Turns out it was in the 50th percentile.
Ultrasounds aren't really that accurate with this sort of thing.
They didn't tell me either. However, my doctor told me babies' heads usually take after their fathers. So if your SO has a big head, chances are, so will your baby. DS's head was 14 inches, 35cm.
Re: How big is your babies head? **UPDATE** to add chart.