Infant head shape differs significantly from that of the adult. In the infant, the cranium is much more elongate and bulbous, with large frontal and parietal (side) prominences. At birth, the circumference of the head is about 13–14 inches. It increases by 17% during the first 3 months of life, and by 25% at 6 months of age. It increases by about 1 inch during the 2nd year, and during the 3rd through the 5th year head circumference increases by about one-half inch per year. There is only a 4 inch increase in herd circumference from the end of the 1st year to the 20th year.
At birth, the brain is typically 25% of its adult size, although the bodyweight of the newborn is only about 5% of adult weight.