<prompt> Microcephaly is defined as head circumference ___1SD for age and sex2SD for age and sex3SD for age and sex4SD for age and sex  <response>: Microcephaly is defined as an occipitofrontal circumference more than 3 standard detions SD below the mean for given age, sex and gestation. Causes are structural brain malformations, inherited syndromes, congenital or acquired infections, hypoxic ischemic insults and rarely metabolic disorders. Isolated inherited microcephaly is autosomal recessive. Reference : page 36 Ghai Essential Pediatrics 9 th edition 