Physical Growth: The Rapid Advances of Infancy

  1. LO1How does the human body develop?

Infants grow at a rapid pace over the first two years of their lives (see Figure 5-1).By the age of 5 months, the average infant’s birth weight has doubled to around 15 pounds. By the first birthday, the baby’s weight has tripled to about 22 pounds. Although the pace of weight gain slows during the second year, it still continues to increase. By the end of his or her second year, the average child weighs around four times as much as he or she did at birth. Of course, there is a good deal of variation among infants. Height and weight measurements, which are taken regularly during physician’s visits during a baby’s first year, provide a way to spot problems in development.

Figure 5-1

Height and Weight Growth

Although the greatest increase in height and weight occurs during the first year of life, children continue to grow throughout infancy and toddlerhood.

The weight gains of infancy are matched by increased length. By the end of the first year, the typical baby grows almost a foot and is about 30 inches tall. By their second birthdays, children average a height of 3 feet.

Not all parts of an infant’s body grow at the same rate. For instance, as we saw first in Chapter 2, at birth the head accounts for one-quarter of the newborn’s entire body size. During the first two years of life, the rest of the body begins to catch up. By age 2 the baby’s head is only one-fifth of body length, and by adulthood it is only one-eighth (see Figure 5-2).

Figure 5-2

Decreasing Proportions

At birth, the head represents one-quarter of the neonate’s body. By adulthood, the head is only one-eighth the size of the body. Why is the neonate’s head so large?

There also are gender and ethnic differences in weight and length. Girls generally are slightly shorter and weigh slightly less than boys, and these differences remain throughout childhood (and, as we will see later in the book, the disparities become considerably greater during adolescence). Furthermore, Asian infants tend to be slightly smaller than North American Caucasian infants, and African American infants tend to be slightly bigger than North American Caucasian infants.

Following are four major principles (summarized in Table 5-1) that govern growth.

Table 5-1

The Major Principles Governing Growth

Cephalocaudal Principle Proximodistal Principle Principle of Hierarchical Integration Principle of the Independence of Systems
Growth follows a pattern that begins with the head and upper body parts and then proceeds to the rest of the body. Based on Greek and Latin roots meaning “head-to-tail.” Development proceeds from the center of the body outward. Based on the Latin words for "near" and “far.” Simple skills typically develop separately and independently. Later they are integrated into more complex skills. Different body systems grow at different rates.