I think our average is 13-14.5. However, if *I* measure the head, the mw measures the head, and the doc measures the head, we'd get three different measurements. Everyone places the tape a little differently, some pull it really taut, others not so much,etc. It also depends upon how soon after the birth you measure. The moulding on the head can change dramatically even in 1/2 an hour after birth. And mode of birth also affects it--if mama had a scheduled c-section, or if baby was breech, or if baby was in a funky position so that mama had to push for a heck f a long time, that would affect the shape of the baby's head.
So while, yes, 15 is a pretty good sized head (I'm sure it's all brains
), the Guiness Book of World Records probably won't be knocking down your door.