Growth and Development

After completing this section of the chapter, you should be able to meet the following objectives:

♦ Characterize the use of percentiles to describe growth
and development during infancy and childhood

♦ Describe the major events that occur during prenatal
development from fertilization to birth

♦ Define the terms low birth weight, small for gestational age,
and large for gestational age

♦ Identify reasons for abnormal intrauterine growth

♦ Describe assessment methods for determination of
gestational age

The phrase growth and development describes a process whereby a fertilized ovum becomes an adult person. Phys­ical growth describes changes in the body as a whole or in its individual parts. Development, on the other hand, em­braces other aspects of differentiation, such as changes in body function and psychosocial behaviors.

Physical growth occurs in a cephalocaudal (head-to-toe) direction. Relative body proportions change over the life span. In early fetal development, the head is the largest part of the body, but proportional size changes as the in­dividual grows (Fig. 2-1).

The average newborn weighs approximately 3000 to 4000 g and is 50 to 53 cm long. The first year is a period of rapid growth demonstrated by lengthening of the trunk and deposition of subcutaneous fat.⁵ After the first year until onset of puberty, the legs grow more rapidly than any other part of the body.

The onset of puberty is marked by significant alter­ations in body proportions because of the effects of the pu-bertal growth spurt. The feet and hands are the first to grow. Because the trunk grows faster than the legs, at ado­lescence a large portion of the increase in height is a result of trunk growth. The brain also undergoes a period of

rapid growth. At birth, the brain is 25% of adult size; at

1 year, it is 50% of adult size; and at 5 years, it is 90% of
adult size. The size of the head reflects brain growth.⁶ Linear
growth is a result of skeletal growth. After maturation of
the skeleton is complete, linear growth is complete. By

2 years of age, the length is 50% of the adult height. Be­
ginning with the third year, the growth rate is 5 to 6 cm
for the next 9 years. A growth spurt during adolescence is
necessary for adult height to be reached. Males add ap­
proximately 20 cm and females 16 cm to height during
this time. Weight is rapidly increased after birth. Generally
by 6 months of age, the birth weight is doubled; by 1 year
of age it is tripled. The average weight increase is 2 to
2.75 kg per year until the adolescent growth spurt begins.⁷

Growth and development encompass a complex in­teraction between genetic and environmental influences. The experience of each child is unique, and the patterns of growth and development may be profoundly different for individual children within the context of what is consid­ered normal. Because of the wide variability, these norms often can be expressed only in statistical terms.

Evaluation of growth and development requires com­parison of an individual's growth and development to a standard. Statistics are calculations derived from measure­ments that are used to describe the sample measured or to make predictions about the rest of the population repre­sented by the sample. Because all individuals grow and de­velop at different rates, the standard must somehow take this individual variation into account. The standard typi­cally is derived from measurements made on a sample of individuals deemed representative of the total population. When multiple measurements of biologic variables such as height, weight, head circumference, and blood pressure are made, most values fall around the center or middle of all the values. Plotting the data on a graph yields a bell-shaped curve, which depicts the normal distribution of these continuously variable values (Fig. 2-2).

The mean and standard deviation are common statis­tics used in describing the characteristics of a population. The mean represents the average of the measurements; it is the sum of the values divided by the number of values. A normal bell-shaped curve is symmetric, with the mean falling in the center of the curve and with one half of the values falling on either side of the mean. The standard de-

viation determines how far a value varies or deviates from the mean. The points 1 standard deviation above and below the mean include 68% of all values, 2 standard de­viations 95% of all values, and 3 standard deviations 99.7% of all values.⁵ If a child's height is within 1 standard devi­ation of the mean, he or she is as tall as 68% of children in the population. If a child's height is greater than 3 stan­dard deviations, he or she is taller than 99.7% of children in the population.

The bell-shaped curve can also be marked by percen-tiles, which are useful for comparison of an individual's val­ues with other values. When quantitative data are arranged in ascending and descending order, a middle value, called the median, can be described with one half (50%) of the val­ues falling on either side. The values can be further divided into percentiles. A percentile is a number that indicates the percentage of values for the population that are equal to or below the number. Percentiles are used most often to com­pare an individual's value with a set of norms. They are used extensively to develop and interpret physical growth charts and measurements of ability and intelligence