Microcephaly is broadly defined as a small head size, typically greater than two standard deviations below normal, as measured via occipital frontal circumference, a method by which a tape measure is used to assess the circumference encompassing the occiput and frontal bones (forehead). It is a clinical sign and not a disease.
Microcephaly has recently gained notoriety in the popular press because of its association with the Zika virus, one of many viruses that may lead to microcephaly by first damaging the brain.  However, causality has not definitely been proven.
The skull is comprised of six bones that are not fused at birth and which begin to fuse over the next few months. Head circumference typically grows because of the underlying brain growing, exerting physical force on the skull bones and causing them to expand as well. The rate of expansion is highest in the first few months and gradually plateaus over time. Various gender-specific growth charts have been published, but the Centers for Disease Control (CDC) currently recommends that children in the United States between the ages of 0 and 2 years old are tracked with the World Health Organization (WHO) growth charts, and with the CDC growth charts after children turn 2 years old. Minor differences do exist between the two charts. Clinicians can easily tell the difference because charts from WHO data tend to stop at 24 months of age, whereas the charts from CDC data extend to 36 months of age. [6, 7]
If the sutures fuse prematurely, the skull may take on an abnormal shape and the head circumference will grow at a much slower rate than age-matched controls. (For further information, please see the Medscape Reference article Pediatric Craniosynostosis.)
Alternatively, if an infant’s brain has a lower volume than normal (i.e., due to a decreased number of neurons and glial cells), it may not grow as much as other infants’ brains that have a higher number of neurons and glial cells. Decreased brain growth would then lead to decreased outward force exerted on the skull, and decreased expansion of the head circumference. This process may result in a normally shaped or symmetric head, as opposed to the abnormal shape that results from craniosynostosis. Various etiologies can lead to decreased brain volume. More commonly, prenatal or perinatal insults to the brain or genetic/metabolic conditions cause poor brain growth. Less commonly, genetic neurodegenerative diseases (e.g., Rett Syndrome) will cause progressive microcephaly in infants and toddlers.
The etiology of microcephaly can be broadly divided into two categories: premature fusion of cranial sutures (i.e., craniosynostosis) or poor brain growth.
Genetic conditions that can cause microcephaly include the following:
Trisomy 21, 13, 18
Cri du Chat syndrome - 5p deletion
Wolf–Hirschhorn syndrome - 4p deletion
Williams syndrome - 7q11.23 deletion
Cornelia de Lange syndrome
Primary autosomal recessive microcephaly (microcephaly primary hereditary [MCPH]) - mutation in the abnormal spindle microtuble assembly (ASPM) gene, which impacts cell division of neural progenitor cells
Maternal deprivation problems can also lead to microcephaly (e.g., folate deficiency, malnutrition, and hypothyroidism).
Postnatal onset microcephaly
Postanatal onset microcephaly can result from inborn errors of metabolism including congenital disorders of glycosylation, mitochondrial disorders, peroxisomal disorders, and Menkes disease. Disruptive injuries such as traumatic brain injury, hemorrhagic and ischemic stroke, and hypoxic-ischemic encephalopathy can also lead to microcephaly.
Microcephaly is not a common condition. State birth defects tracking systems have estimated that microcephaly ranges from 2 babies per 10,000 live births to about 12 babies per 10,000 live births in the Unites States.  Two large population-based studies have found similar incidences of microcephaly in newborns: 0.56% and 0.54%, respectively. [1, 2]
Similar studies for postnatal onset microcephaly could not be found and are not mentioned in the American Academy of Neurology’s (AAN) Practice Parameter: Evaluation of the child with microcephaly (an evidence-based review). 
Microcephaly itself does not necessarily predict someone’s functional outcome but can be associated with many other clinical problems, depending on the etiology of the microcephaly. For example, seizures, cerebral palsy, developmental delay, hyperactivity, and other neurodevelopmental problems occur more commonly in children with microcephaly.