The definition of an abnormally large or small head is based on statistics and is arbitrary.  A normal child can have a head circumference which is borderline in either direction.  The size of the infant's head is determined by the growth of the cranial contents and the structure of the cranial bones.  The rate of growth is influenced by heredity, genral health, fucntion of other systems (cardiac and encdocrine) and the child's age, weight and race.  A growth curve can document the progress of thechild and reveal any deviation from expected growth.

A measurement that is considered abnormal should be supplemnted by observationb and measurement of parents and siblings.  It has been documented, that we Guatemalans, have smaller heads comapred to the NCHS statistics and the importance here is to keep a close track to determine progressinon of growth, that should be parallel to the known NCHS charts.