The standard head circumference for adults is 54-58 cm. Head circumference is a value that reflects the normal development of human brain and skull. Head circumference is measured by measuring the circumference from the protruding part above the eyebrow arch as the starting point and passing through the posterior occipital node to the starting point for one week, which is most meaningful when children are two years old. A large head circumference is commonly associated with diseases such as hydrocephalus and rickets; a small head circumference is commonly associated with diseases such as microcephaly. The average head circumference of a newborn is 34 cm at birth, and can reach an average of 46 cm at age 1, 48 cm at age 2, and close to adult head circumference at age 15.