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This chapter provides an overview of typical development, identifies developmental variations, and discusses several developmental disorders. The chapter does not cover typical development in the newborn period or adolescence (see Chapters 2 and 4, respectively). It addresses behavioral variations that reflect the spectrum of normal development, along with developmental and behavioral disorders and their treatment. The developmental principle of ongoing change and maturation is integral to the daily practice of pediatrics. It is the basic science of pediatrics. For example, we recognize that a 3-month-old infant is very different from a 3-year-old toddler or a 13-year-old adolescent, not only with respect to what the child can do but also in terms of the kinds of illness he or she might have. From the perspective of the general pediatrician, all these areas should be viewed in the context of a “medical home.” The medical home is defined as the setting that provides consistent, continuous, culturally competent, comprehensive, and sensitive care to children and their families. It is a setting that advocates for all children, whether they are typical or have developmental challenges or disabilities. By incorporating the principles of child development—the concept that children are constantly changing—the medical home is the optimum setting to understand and enhance typical development and to address variations, delays, and deviations as they may occur in the life trajectory of the child and the family.


Typical children follow a trajectory of increasing physical size and increasing complexity of function, especially during the first five years of life. The child triples his or her birth weight within the first year and achieves two-thirds of his or her adult brain size by age 2½–3 years of age. The child progresses from a totally dependent infant at birth to a mobile, verbal person who is able to express his or her needs and desires by age 2–3 years. In the ensuing 3 years the child further develops the capacity to interact with peers and adults, achieves considerable verbal and physical prowess, and becomes ready to enter the academic world of learning and socialization.

It is critical for the clinician to identify disturbances in development during these early years as there are windows of opportunity or sensitive periods when appropriate interventions may be instituted to effectively address developmental challenges.


From a motor perspective, children develop in a cephalocaudal direction. They can lift their heads with good control at 3 months, sit independently at 6 months, crawl at 9 months, walk at 1 year, and run by 18 months. The child learning to walk has a wide-based gait at first. Next, he or she walks with legs closer together, the arms move medially, a heel-toe gait develops, and the arms swing symmetrically by 18–24 months.

Clinicians often focus on gross motor development, but ...

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