In infancy, growth is so fast, and the consequences of neglect are so severe, that gains need to be closely monitored. Medical checkups, including measurement of height, weight, and head circumference, occur every few weeks at first. Body Size
Exactly how rapidly does growth typically occur? We saw in Chapter 4 that at birth the average infant weighs 71⁄2 pounds (3,400 grams) and measures about 20 inches (51 centimeters). This means that the typical newborn weighs less than a gallon of milk and is about as long as the distance from a man’s elbow to the tips of his fingers. Infants typically double their birth weight by the fourth month and triple it by their first birthday. Physical growth slows in the second year, but it is still rapid. By 24 months most children weigh almost 30 pounds (131⁄2 kilograms) and are between 32 and 36 inches (81–91 centimeters) tall. This means that typical 2-year-olds are already half their adult height. They are also about 15 to 20 percent of their adult weight, four times as heavy as at birth. Each of the above numbers is a norm, an average or standard for a particular population. Norms must be carefully interpreted. The “particular population” for the norms above is a representative sample of North American infants, who may be unlike representative samples of infants from other regions of the world. To understand norms, you also need to understand percentiles. A child who is average is at the 50th percentile, a number that is midway between 0 and 100, with half of the children above it and half below it. Percentiles allow a child’s growth to be compared not only with that of other children but also with his or her own prior development.
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