The Future of Children, Princeton - Brookings: Providing research and analysis to promote effective policies and programs for children.

Some of the most impressive developmental accomplishments of the early years are the most visible. The young child grows faster during the first three years than he or she ever will again.² Not only does the child grow physically larger but body proportions also change. The top-heavy newborn evolves into a five-year-old with a body more closely resembling that of an adult. These changes in body proportions (together with the remarkable advances in brain development that integrate neural pathways governing behavior) help to account for the striking changes in motor coordination, balance, and dexterity that also characterize the early years. The physically uncoordinated newborn learns to sit up by six months of age, stand and walk shortly after the first birthday, and (impatiently, exuberantly, or anxiously) jump in place by the second birthday. The rudimentary grasping reflex of infants evolves into more sophisticated, deliberate eye-hand coordination that enables them to pick up small objects (such as a pea on a dinner plate) by the end of the first year. By age two, toddlers are using their hands to build towers, and by age three, to draw circles on paper.

These physical advances are also fostered by growth in sensory acuity. Because of changes in the eye, ear, and other sensory organs, and developments in brain organization, infants quickly learn to scan the visual field and to discriminate sounds in much more sophisticated ways. And there are other changes in the young child that derive from the growth of the body. Parents welcome the greater regularity of sleep-wake cycles, the diminishing of crying and unexplained fussiness, and the enhanced predictability in mood that derive from rapid growth in neurobehavioral organization.

There is a tendency in this culture to attribute these remarkable physical achievements to an inborn maturational timetable. Often overlooked is the extent to which these accomplishments rely on crucial catalysts from experience and the environment. But it is a truism of development that the periods of most rapid advance are often periods of greater vulnerability because of the many changes that occur in a short span of time. The rapid growth of the body is metabolically demanding, for example, which means that a nutritionally adequate diet is one of the most crucial requirements for healthy early physical growth. Deficiencies in iron and vitamins owing to chronic undernutrition in the early years can result in cognitive delays, listlessness, and diminished resistance to disease.³ Young children are also vulnerable to exposure to infectious diseases, drugs and other controlled substances, and environmental toxins (like lead-based paint). In children whose developing physical systems are still maturing, such exposure can result in more profound harm than if it occurs at a later age. Moreover, accidents are a leading cause of injury and death for the very young, owing to children's characteristically poor judgment about potentially dangerous circumstances.

Consequently, healthy physical development in the early years hinges critically on caregivers' determination to protect young children from the harms that might occur. This includes efforts to ensure a healthy, adequate diet; timely immunizations; early vision and hearing screening to detect and correct sensory deficits before they endure; regular health care; and efforts to monitor children's safety in a physical environment that is friendly to the needs and interests of young children.