Physical growth usually refers to changes in size or mass; so it is correct to say that a child grows in stature (height) or body weight. Even though most people usually think of growth at the level of the whole child, the cells and internal structures that make up the child also grow, primarily by increasing in number or size. Consequently, auxologists (those who study child growth) may be interested in the growth of bones to help understand fractures and osteoporosis; the growth of the heart walls to help understand hypertension (high blood pressure) and heart disease; or the growth of adipose tissue (body fat) to help understand obesity.
The measurement of body dimensions such as those used in growth studies is called anthropometry. Past growth is usually measured as the size attained at a chronological age, for example the weight of a child at eight years old. Assessment of the rate of growth requires that a body dimension, such as weight, be measured twice over a period, and then the change is expressed in terms of the increment or velocity of growth, for example in pounds or kilograms per year.
Some physical changes in childhood are more complicated than just size or mass. These changes include alterations in body structures and functions and can be termed development. Physical developmental changes are as diverse as the closing of the fontanels (soft spots) in a baby's skull, the erupting of teeth, learning to walk, or the deepening of the voice of boys during adolescence.
Some developmental changes are considered maturational, or indicators of physical maturity. Maturation is the progression of developmental changes toward the characteristics of adults. Physical maturation occurs from the time of conception, but some of the most commonly recognized indicators of maturation become apparent during adolescence. Changes in body shape, breast development in girls, pubic hair development in both genders, and development of facial hair in boys are visible indicators of maturation toward adult appearance of the body, and they signal adult reproductive functioning. The cessation of the growth of long bones, associated with the final attainment of adult stature is also a maturational event.
Although growth and maturation are certainly related, distinguishing between them is important because some physiological and hormonal processes affect growth and maturation differentially, as do some diseases. It is easy to observe that children of the same size can differ in maturational status and that fully mature individuals (adults) can be of different sizes.
Growth differences between males and females begin before birth and continue until adulthood. Generally, boys are larger than girls throughout gestation, so that when they are born at full term (forty weeks), male newborns usually weigh about 150 grams (5.3 ounces) more than females, and are about one centimeter (0.4 inches) longer. Even though they are smaller than their male counterparts, female babies are usually more mature skeletally and neurologically at birth.
After birth, most body dimensions, such as stature, body circumferences, and weight, follow a similar pattern of growth: a period of very rapid growth in infancy, slower growth during middle childhood, a very rapid growth phase or spurt in adolescence, and a period of rapidly decelerating growth, ending with adult size. Obviously, some body dimensions, such as weight or fatness, can continue to change throughout adulthood. The different phases of postnatal growth can be appreciated more easily by looking at the rates of growth, or velocity, in addition to attained size.
On average, boys are taller and heavier than girls at every postnatal age, except from about nine to thirteen years. The reversal of size differences at these ages results from girls entering their adolescent growth spurt about two years earlier than boys. Boys usually end up about nine to thirteen centimeters (three to five inches) taller and seven to nine kilograms (fifteen to twenty pounds) heavier than girls at eighteen years of age. This is primarily because boys grow approximately two years longer than girls do before their spurt, and because the spurt of boys usually is more intense and lasts a little longer than that of girls.
Timing of Maturation
Different body structures and functions often mature at differing rates, and they achieve adult status at different average chronological ages. For example, the three tiny bones of the inner ear (the incus, malleus, and stapes) are mature before birth, while the last bone to achieve adult status (the clavicle or collarbone) does not do so until approximately twenty-five years of age.
Even within groups of healthy children, there is considerable variation in the timing of the same maturational processes and events. For example, the first menstrual period of girls, or menarche, signals achievement of one aspect of adult reproductive functioning and is a widely used maturational indicator. (The corresponding but less noticeable event in boys is the first production of sperm cells, or spermarche.) The average age at menarche for girls in the United States is approximately 12.8 years of age. About two-thirds of U.S. girls will attain menarche within one year of the average timing, and about 98 percent of all girls within two years. For healthy girls, this variation in the timing of menarche is due to inherited patterns from their parents. Age at menarche (and most other maturational timing) can be delayed by malnutrition and infectious disease, and less commonly by hormonal dysfunction.
The chronological age at which maturational events occur provides a measure of the relative timing of that event in the child's growth and development. In addition to menarche, other examples of maturational events whose timing may be of interest include onset of ossification of bony centers (visible in X rays), eruption of teeth, first walking, first appearance of pubic hair, the age when the adolescent spurt is at its peak velocity, and the final fusion of the growing centers of long bones.
Of course, these maturational events are really biological processes that occur progressively in the developing child and the "event" is really just an arbitrary point in the developmental process that has been defined by auxologists so that it can be measured more easily. Some maturational processes have been more or less arbitrarily defined in stages or grades so that the progress through the stages can be measured. The progressive development of the secondary sexual characteristics associated with sexual maturation is a common example where such stages have been applied. The development of breasts in girls, penis and scrotum in boys, and pubic hair in both genders have carefully described stages of development that pediatricians and endocrinologists use clinically and that are also used by researchers who are interested in normal and abnormal adolescent growth and maturation.
Nutrition, Health, and the Environment
Physical growth and maturation are often used as indicators of child health because they are sensitive to nutritional deficiencies, infection, and poverty. Growth is a very adaptable process that will slow in the face of extreme nutritional deficiency, for example, as a mechanism to conserve nutrients for body functions essential to the child's survival. Growth will resume or even catch up at faster rates than normal when the nutritional deficits are remedied. This sensitivity to health and environmental constraints makes growth an excellent indicator of the adequacy of nutrition and the health of individuals and of populations. As basic indicators of health, pediatricians compare the attained stature and weight of children and their rates of growth with the expected values for healthy children or with growth standards.
In public-health studies comparing different populations or countries, the percentage of young children with very short stature (stunting) and the percentage of those whose weight is very low for how tall they are (wasting) are important indicators of nutritional and health conditions affecting children. In such studies, the average age at menarche, or of other maturational events, may be used to indicate the adequacy of general health and nutritional conditions.
Some examples of the average age at menarche from different countries are given in Table 1. Average ages of menarche greater than 13.5 years are usually considered to be associated with some general nutritional or health constraints in the country. In the case of Nepal, these issues are probably complicated by the people living at very high altitude, which may affect growth and maturation because of the reduced availability of oxygen to the body.
When nutritional energy (calories from food) is in excess of what the body uses and what is expended in physical activity, it is stored in adipose tissue. This fat tissue is accumulated within the body and subcutaneously (under the skin). The growth in weight of children and measurements of the thickness of the subcutaneous fat by calipers are used as indicators of overweight and obesity. Sometimes the weight of children is expressed as an index relative to stature (calculated by dividing the weight, in kilograms, by the square of stature, in meters) to yield the body mass index (BMI). BMI standards are also commonly used to define overweight and obesity and to relate these conditions to various health outcomes.
Physical growth includes many aspects of the biological development of children that can reflect genetics, nutrition, health, and the environment. The aspects of physical growth are central to the child's progress toward adulthood, and they inevitably interact with psychological, behavioral, and social aspects of the developing child.
Buckler, J. M. H. A Reference Manual of Growth and Development, 2nd edition. Oxford: Blackwell Science, 1997.
Eveleth, Phyllis B., and James M. Tanner. Worldwide Variation inHuman Growth, 2nd edition. Cambridge, Eng.: Cambridge University Press, 1990.
Himes, John H., ed. Anthropometric Assessment of Nutritional Status. New York: Wiley, 1991.
Malina, Robert M., and Claude Bouchard. Growth, Maturation, and Physical Activity. Champaign, IL: Human Kinetics, 1991.
Tanner, James M. Foetus into Man: Physical Growth from Conception to Maturity. Cambridge, MA: Harvard University Press, 1990.
Tanner, James M., R. H. Whitehouse, and M. Takaishi. "Standards from Birth to Maturity for Height, Weight, Height Velocity, and Weight Velocity: British Children, 1965." Archives of Disease in Childhood 41 (1966):613-635.