Familial variation of head size and adjustment for parental head circumference.

Occipitofrontal head circumference was measured on 122 twin pairs, their spouses, and their children. These data indicate that approximately 50% of normal head size variation is familial. Because of the relationship between the head size of normal children and their parents, adjustment of a child's head size value by the average parental value permits better definition of the range of normalcy. A method is presented that will allow physicians to make this adjustment, providing a more refined assessment of head size when there is a suspected abnormality.