Microcephaly is defined as a:

Ans. A. Head circumference more than 2 SD above the mean for age and sexMacrocephaly is defined as head circumference more than two standard deviations (SD) above the mean value for a given age and gender. It has to be differentiated from megalencephaly, which is defined as increase in the size of the brain parenchyma. Head circumference is measured to monitor head growth in infants and children. It is also known as occipitofrontal circumference (OFC) and it denotes the size of the cranium. Macrocephaly can result from enlargement of the skull bones or an increase in the volume of the intracranial structures like cerebrospinal fluid, blood, or the brain parenchyma itself (megalencephaly). It may be secondary to raised intracranial pressure or space-occupying lesions. It can also be a feature of various congenital syndromes and is then referred to as Syndromic macrocephaly. The pathogenesis is therefore cause-dependent.