Head circumference below 2 standard deviations below the mean for age and gender at birth. The phenotype is Primary microcephaly (Human Phenotype Ontology id HP_0011451). Also known as: Congenital decreased head circumference, Congenital microcephaly, Congenital small skull, Decreased head circumference present at birth, Microcephaly present at birth, Small cranium present at birth.