Primary autosomal recessive microcephalies (MCPH) and Seckel syndrome (SCKS) spectrum disorders are characterized by microcephaly and the absence of visceral malformations. Although MCHP and SCKS were previously distinguished by height (maximum height in SCKS was equivalent to the minimum height in MCPH), stature is no longer a discriminating feature, leading to the conclusion that these phenotypes constitute a spectrum rather than distinct entities. Microcephaly is characterized by: Onset during the second trimester of gestation; Occipito-frontal head circumference (OFC) at birth equal to or less than -2 SD below the mean for sex, age, and ethnicity; Slower than average increase in OFC after birth. Variable findings in the MCPH-SCKS spectrum disorders include: Brain structure (which is normal in the majority); Degree of cognitive impairment (usually mild to moderate without significant motor delay in the majority of persons with MCPH and more severe in those with SCKS and MCPH with brain malformations); Degree of short stature; Craniosynostosis (which may be secondary to poor brain growth).