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Child development is a dynamic interaction of biologic and environmental factors. Typically, over a period of many years, an infant who is initially entirely dependent on others to meet life-sustaining needs such as food and shelter develops into an independent adult capable of caring for others. Growth of a child’s skills occurs across multiple domains of development, including motor, language, cognitive, and socioemotional domains, and allows for slowly increasing independence. Within any single domain, the sequence of new skills acquired is fairly consistent, but there is significant individual variability in the rate of development within and across the different domains that complicates attempts to distinguish children with normal variations in development from children with developmental delays. However, early detection of developmental delays is essential to promote early intervention that can improve developmental outcomes and family adjustment. During well-child care, clinicians are the professionals most likely to interact with a child early in development and thus have a unique opportunity and responsibility to detect children with developmental delays. The approach to the evaluation of developmental delay is discussed further in Chapters 185 and 547.


The sequences in which children gain new skills in motor, language, cognitive, and socioemotional development are precisely delineated, and for many skills, the age at which 50% of children will be able to accomplish a specific task is known. A child’s developmental age is defined as the age at which approximately 50% of children would demonstrate similar functioning. Since developmental age is determined on the basis of the child’s functioning, it could be higher or lower than the chronological age.


A global developmental delay occurs when a child’s developmental age lags behind chronological age across all the domains of development. The degree of delay is often quantified by calculating the developmental quotient (DQ = developmental age/chronological age × 100). When a child is developing at the expected rate, the DQ is 100. However, if a 2-year-old child is functioning at a 1-year-old developmental age, the child’s DQ is 50, indicating that the child is developing at half the expected rate. A DQ less than 75 to 80 represents a significant developmental delay. When skills in one domain lag behind (or are more advanced than) skills in other domains, a dissociation in the domains of development occurs, such as seen with language disorders, coordination disorders, and learning disabilities.


Typically, a child’s developmental age is similar across the different domains of development. However, when skills in 1 domain lag behind (or are more advanced than) skills in other domains, a dissociation in the domains of development occurs. Disassociations occur in situations such as language disorders, coordination disorders, and learning disabilities.


Intellectual disability (formerly mental retardation) is defined as a significant deficit in both cognitive skills and adaptive functioning that develops prior to 18 years of age.1 The deficit in cognitive skills is determined by the intelligence quotient (IQ) on a standardized IQ ...

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