At birth, the average head circumference of boys is 33-34 cm and 46 cm at the age of 1. In the second year after birth, the growth rate of head circumference slows down to about 2 cm, and the head circumference is about 48 cm at the age of 2. From 2 to 15 years old, the head circumference only increases by 6-7 cm, i.e., to 54-58 cm, after which the head circumference generally does not grow again. However, due to genetic factors and individual development, each individual varies greatly and the cranial shape is not exactly the same, so the head circumference data will vary. If a boy has no other accompanying symptoms or deformities, a slightly larger or smaller head circumference does not necessarily indicate a problem, and should be judged in the context of individual development. If the head circumference is more or less outside the normal range, it may indicate the presence of certain diseases. Generally speaking, head circumference measurements are most valuable within the age of 2 years to reflect the development of the skull. Diseases such as hydrocephalus and rickets need to be considered when a boy’s head circumference is too large, often accompanied by the sunset phenomenon, in which the eyes look down and the pupils resemble the sun about to set due to increased intracranial pressure pressing on the eyeballs. Small head circumference is commonly associated with microcephaly, which is related to premature closure of fontanelle and is often associated with mental retardation.