General Patterns
Growth differences between males and females
begin before birth and continue until adulthood.
Generally, boys are larger than girls throughout ges-
tation, 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 ba-
bies are usually more mature skeletally and
neurologically at birth.
After birth, most body dimensions, such as stat-
ure, body circumferences, and weight, follow a similar
pattern of growth: a period of very rapid growth in in-
fancy, slower growth during middle childhood, a very
rapid growth phase or spurt in adolescence, and a pe-
riod 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 thir-
teen 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 kilo-
grams (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 be-
fore 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 ma-
ture 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, malle-
us, and stapes) are mature before birth, while the last
bone to achieve adult status (the clavicle or collar-
bone) 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 mat-
urational processes and events. For example, the first
menstrual period of girls, or menarche, signals
achievement of one aspect of adult reproductive func-
tioning 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 varia-
tion in the timing of menarche is due to inherited pat-
terns from their parents. Age at menarche (and most
other maturational timing) can be delayed by malnu-
trition 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 matura-
tional 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 cen-
ters of long bones.
Of course, these maturational events are really bi-
ological processes that occur progressively in the de-
veloping 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 mea-
sured 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 sec-
ondary 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 devel-
opment 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 nu-
tritional 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 chil-
dren or with growth standards.PHYSICAL GROWTH 307