Secondary microcephaly (Human Phenotype Ontology identifier HP_0005484) is head circumference which falls below 2 standard deviations below the mean for age and gender because of insufficient head growth after birth. Also known as: Acquired microcephaly, Deceleration of head growth, Microcephaly, acquired, Microcephaly, postnatal, Postnatal deceleration of head circumference, Postnatal microcephaly.