It has been classified in two types based on the onset:

Congenital
Isolated
Familial (autosomal recessive) microcephaly
Autosomal dominant microcephaly
X-linked microcephaly 
Chromosomal (balanced rearrangements and ring chromosome)
Syndromes
Chromosomal
Poland syndrome
Down syndrome
Edward syndrome
Patau syndrome
Unbalanced rearrangements
Contiguous gene deletion
4p deletion (Wolf–Hirschhorn syndrome)
5p deletion (Cri-du-chat)
7q11.23 deletion (Williams syndrome)
22q11 deletion (DiGeorge syndrome)
Single gene defects
Smith–Lemli–Opitz syndrome
Seckel syndrome
Cornelia de Lange syndrome
Holoprosencephaly
Primary microcephaly 4
Wiedemann-Steiner syndrome
Acquired
Disruptive injuries
Ischemic stroke
Hemorrhagic stroke
Death of a monozygotic twin
Vertically transmitted infections
Congenital cytomegalovirus infection
Toxoplasmosis
Congenital rubella syndrome
Congenital Varicella Syndrome
Zika virus (see Zika fever#Microcephaly) 
Drugs
Fetal hydantoin syndrome
Fetal alcohol syndrome
Other
Radiation exposure to mother
Maternal malnutrition
Maternal phenylketonuria
Poorly controlled gestational diabetes
Hyperthermia
Maternal hypothyroidism
Placental insufficiency
Craniosynostosis

Postnatal onset
Genetic
Inborn errors of metabolism
Congenital disorder of glycosylation
Mitochondrial disorders
Peroxisomal disorder
Glucose transporter defect
Menkes disease
Congenital disorders of amino acid metabolism
Organic acidemia
Syndromes
Contiguous gene deletion
17p13.3 deletion (Miller–Dieker syndrome)
Single gene defects
Rett syndrome (primarily girls)
Nijmegen breakage syndrome
X-linked lissencephaly with abnormal genitalia
Aicardi–Goutières syndrome
Ataxia telangiectasia
Cohen syndrome
Cockayne syndrome
Acquired
Disruptive injuries
Traumatic brain injury
Hypoxic-ischemic encephalopathy
Ischemic stroke
Hemorrhagic stroke
Infections
Congenital HIV encephalopathy
Meningitis
Encephalitis
Toxins
Chronic kidney failure
Deprivation
Hypothyroidism
Anemia
Congenital heart disease
MalnutritionGenetic mutations cause most cases of microcephaly. Relationships have been found between autism, duplications of genes and macrocephaly on one side. On the other side, a relationship has been found between schizophrenia, deletions of genes and microcephaly. Several genes have been designated "MCPH" genes, after microcephalin (MCPH1), based on their role in brain size and primary microcephaly syndromes when mutated. In addition to microcephalin, these include WDR62 (MCPH2), CDK5RAP2 (MCPH3), KNL1 (MCPH4), ASPM (MCPH5), CENPJ (MCPH6), STIL (MCPH7), CEP135 (MCPH8), CEP152 (MCPH9), ZNF335 (MCPH10), PHC1 (MCPH11) and CDK6 (MCPH12).
It is reported that in the United States, microcephaly occurs in 2 to 12 babies per 10,000 births. Signs and symptoms
There are a variety of symptoms that can occur in children. Infants with microcephaly are born with either a normal or reduced head size. Subsequently, the head fails to grow, while the face continues to develop at a normal rate, producing a child with a small head and a receding forehead, and a loose, often wrinkled scalp. As the child grows older, the smallness of the skull becomes more obvious, although the entire body also is often underweight and dwarfed.Severely impaired intellectual development is common, but disturbances in motor functions may not appear until later in life. Affected newborns generally have striking neurological defects and seizures. Development of motor functions and speech may be delayed. Hyperactivity and intellectual disability are common occurrences, although the degree of each varies. Convulsions may also occur. Motor ability varies, ranging from clumsiness in some to spastic quadriplegia in others. Causes
Microcephaly is a type of cephalic disorder.