Measurement of the circumference of the child’s head at its largest area (above the eyebrows and ears and around the back of the head). During routine check-ups, the distance is measured in centimeters and compared to previous measurements; normal ranges are based on sex and age (weeks, months).

Measurement of the head circumference is an important part of the routine well baby care. Through years of collecting measurements, values for normal growth rates of infants and young children's heads have been obtained. During the well baby exam a deviation from the expected normal head growth may alert the doctor of a possible problem.

For example a head that is larger than normal or whose size is increasing faster than normal may be an indication of increased intracranial pressure (there are multiple causes for this condition including head injury, meningitis, water on the brain, or a bleed). An exceptionally small head size (called microcephaly) or too slow of growth rate may indicate that the brain is not developing properly.