Investigating microcephaly.

1. Microcephaly is a clinical finding, not a 'disease', and is a crude but trusted assessment of intracranial brain volume. 2. Developmental processes reducing in utero neuron generation present at birth with 'Primary microcephaly'. 3. 'Secondary microcephaly' develops after birth and predominantly reflects dendritic or white matter diseases. 4. Microcephalic conditions have a heterogeneous aetiology, but increasingly genomic tests are available that allow an exact diagnosis.