It is the result of a permanent interaction between genetic information and environmental factors.
Genes are involved in all stages of development: formation of the neuronal tube, destruction of neurons, migration, differentiation, neuron-glia interactions. (J. Mancini, 2015).
Neurodevelopmental disorders comprise a heterogeneous group of related chronic disorders that manifest in early childhood; They share an alteration in the acquisition of cognitive, motor, language and/or social skills that have an impact on the personal, social and academic.
Neurodevelopment is related not only to genetics but also to the environment of stimulation and affectivity that surrounds the child, obtaining the maturation of the nervous system with the consequent development of brain functions and at the same time the formation of personality. (Maria del Pilar Medina Alva MDP, 2015) They also refer that the formation of the brain occurs from an early age and continues several years after birth. The stages of brain development occur in: neuronal proliferation, migration, organization and lamination of the brain and myelination.
The DSM-V (Association, 2104) has classified them as follows:
intellectual disabilities
Intellectual disability (intellectual development disorder)
Intellectual disability (intellectual developmental disorder) is a disorder that begins during the developmental period and includes limitations in intellectual functioning as well as adaptive behavior in the conceptual, social, and practical domains. The following three criteria must be met:
- Impairments in intellectual functions, such as reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience, confirmed by clinical evaluation and individualized standardized intelligence tests.
- Deficiencies in adaptive behavior that result in failure to meet developmental and sociocultural standards for personal autonomy and social responsibility. Without ongoing support, adaptive deficits limit functioning in one or more activities of daily living, such as communication, social participation, and independent living in multiple settings such as home, school, work, and the community.
- Onset of intellectual and adaptive deficiencies during the developmental period.
Note: The term intellectual disability diagnosis is equivalent to the ICD-11 diagnosis of intellectual development disorders. Although the term intellectual disability is used throughout the manual, both terms are used in the title to facilitate the relationship with other classification systems. In addition, a United States federal clause (Public Law 111-256, Rosa’s Law) substitutes the term mental retardation for intellectual disability, and research journals use the term intellectual disability. Thus, intellectual disability is the term commonly used in the medical, educational and other professions, as well as in public legislation and lobbyists.
Specify current severity
Mild
Moderate
Serious
Deep
global developmental delay
This diagnosis is reserved for individuals younger than 5 years of age when the level of clinical severity cannot be reliably assessed during early childhood. This category is diagnosed when an individual does not meet expected developmental milestones in various domains of intellectual functioning, and applies to individuals in whom a systematic assessment of intellectual functioning cannot be performed, including children too young to participate in standardized tests. This category should be reassessed after a period of time.
Intellectual disability (intellectual developmental disorder), unspecified
This category is reserved for individuals older than 5 years of age when assessment of the degree of intellectual disability (intellectual developmental disorder) by locally available procedures is difficult or impossible due to associated sensory or physical impairments, such as prelingual blindness or deafness, locomotor disability, or the presence of of serious behavior problems or the concurrent existence of a mental disorder. This category will only be used in exceptional circumstances and should be reassessed after a period of time.

Bibliography
Association, A. P. (2104). Guía de Consulta de los Criterios Diagnósticos del DSM-5. Washintong: APA.
J. Mancini, M. M. (2015). Desarrollo neurológico. EMC Pediatría, volúmen 50 número 2 1-11.
María del Pilar Medina Alva MDP, I. C. (2015). Neurodesarrollo Infantil: Características normales y signos de alarma en el niño menor de cinco años. Rev. Perú Med. Exp. Salud Pública, 32 (3) 565:73.