I saw an article this week saying that humans are born with brains 25% of the adult size where as most animals are born with brains 80% of the adult size. It was in a la leche league magazine, and their point was that humans should breast feed until the child is 2 or 3 and catches up with the rest of the animal kingdom on brain maturity. No mention was made of the relationship of brain size to body size or the fact that compared to the rest, humans have ENORMOUS brains. Perhaps they should've gone with some ratio of brain size to body mass. Or brain size to milestone, but that's confusing since so many mammals are born walking (one up on us) and blind (one down).
One more note on brain size. Jackson was born with a such a sweet little heart shaped face. Now he has a long oval head. Two reasons--first, he's been growing extra chins, and second, the rapid brain growth and development has stretched his forehead.