The average head circumference of a newborn is about 34 cm. during the first 3 months and the last 9 months of the first year, the head circumference grows by about 6 cm, so the head circumference is 46 cm at 1 year of age. in the second year after birth, the head circumference growth slows down, and the head circumference is about 48 cm at 2 years of age, 50 cm at 5 years of age, and will be close to the adult head circumference of 54-58 cm at 15 years of age. head circumference measurement is most valuable before 2 years of age, and oversized head circumference is commonly seen in hydrocephalus and post-rickets. A small head circumference is indicative of poor brain development. Head circumference is measured with a soft tape measure flush with the upper edge of both eyebrows, followed by the length of the occipital tuberosity around the head for a week. Its size is closely related to the development of the brain.