Postnatal Growth In Human Birth To Maturity

Growth in Infancy and Childhood: A Pediatric Approach

Horacio Lejarraga, M.D., Ph.D.

Service of Growth and Development, Hospital Garrahan, Buenos Aires, Argentina

At birth the infant is delivered into a postnatal environment characterized by extremely varied and changing conditions. He or she will be subject to intense and continuous physiological demands, which will require consequent adaptive responses during the first years of life. Growth evolving under these circumstances becomes a central subject in pediatric practice, an important objective in child health programs, a relevant health indicator, an instrument for pediatric surveillance in health centers, and an operative issue for health education in the community.

Infancy is a high velocity, rapidly decelerating, nutrition-dependent phase of growth, followed by a growth-hormone-dependent phase of growth, evolving with a slowly decelerating growth velocity during the preschool and school years. Both phases can be expressed by different mathematical functions. At birth, physical size is still strongly related to prenatal growth, and the size of the newborn may not express the size genetically determined by the parents. During the first 2 years, the genes expressing parental size become activated and some children may shift linear growth, changing centiles on distance charts, until they achieve, at around the second year, their genetically determined location on the centiles; that is, canalization.

Infancy and childhood are sensitive periods in human life. Interference with the growth process in early years may have long-term consequences for adult health. Pediatric surveillance and the promotion of normal growth in infancy and childhood includes the knowledge of its physiological basis and the skills for the selection and performance of adequate anthropometric measurements. A correct growth assessment that allows early recognition of growth alterations and identifies abnormal cases is essential for a reasonable clinical orientation of the underlying conditions.

introduction

At birth, the baby leaves the uterus, where he or she lived in a protected environment with quite restricted physiological variations, and enters the postnatal environment, in which it is to develop under extremely varied and varying environmental conditions. Under these circumstances, infancy evolves under a continuous compensation to new levels of stress, "adjustment to striking environmental changes,"1 and the continuous activation and maturation of adaptive mechanisms. Profound developmental changes in the central nervous system take place during infancy and the preschool years, providing a physiological basis for psychomotor development, including the most important intellectual adventure of the human being: the acquisition of language. During the school years and before puberty, the child acquires a large proportion of its cultural inheritance and undergoes an important part of the process of socialization. Infancy and childhood are sensitive periods in human life. Biological, psychological, and social experiences in infancy and childhood have relevant long-term consequences.

the growth curve

Distance and velocity curves for the height of a normal boy are shown in Figure 2-1. Other normal children would have growth curves of the same shape. They may differ in absolute height or growth velocity, the pubertal growth spurt may be experienced earlier or later, but the shape of the curve is always the same. This shape is a primate characteristic (see Chapter 14).

Infants grow very fast during the first year of life, at approximately 25 cm/yr; and during the first half of this year, velocity may be even faster, around 30 cm/yr. A rather steep and continuous deceleration can be observed form birth up to the third year. Thereafter, there is a much milder decay in velocity during school years before the adolescent growth spurt. During this period, mean peak height velocity is approximately 9.5 cm/yr in boys and 8.5 cm/yr in girls.

A small sex difference is present from birth that reaches about 1.0 cm in favor of boys by 5 years of age. The adult sex difference of 12.5-13.0 cm develops gradually prior to and during the pubertal growth spurt (see Chapter 1). For many purposes, the velocity curve can be more informative than that for distance. Three mayor phases of the growth curve can be identified: a rapidly decelerating phase, from birth to approximately 2-3 years; a slowly decelerating phase, from 3 years up to the start of the adolescent growth spurt; and the adolescent growth spurt itself. In this chapter we are concerned with the first and second phases.

Height Velocity Curves
figure 2-1 Distance (A) and velocity (B) curves of height of a normal boy.

infancy and childhood

A Model for Growth in Height

Although there are many mathematical ways of modeling the human growth curve, one is particularly relevant to the purpose of this chapter. Three phases of growth—infancy, childhood, and puberty—are identified and modeled by mathematical expressions. Johan Karlberg2 designed this mathematical model, called the infancy-childhood-puberty (ICP) model. Figure 2-2 shows the decomposition of the growth curve form birth to maturity into the three parts.

The infancy component is expressed with an exponential function that describes growth as rapidly decelerating:

The childhood component is a second degree polynomial function that describes height velocity as following a gradually decelerating course that continues until the end of growth:

The puberty component is modeled with a logistic expression describing additional growth induced by pubertal hormones (gonadotrophins and growth hormone), which produce an acceleration up to peak height velocity and then a deceleration until the end of growth:

Karlberg's ICP model has the advantage of reflecting the main physiological features of the growth process and being compatible with their underlying endocrine and biological influences. The infancy component of the curve seems to start during fetal life, at approximately mid-gestation, and continue with a decelerating trend up to about 3-4 years. This infancy curve is strongly modulated by nutritional factors, to the point that this part of the growth curve has been called the nutrition-dependent phase of growth. In children with congenital hormone deficiency, height at birth and during the first year of life is not as impaired as in malnourished children. The second (childhood) component starts toward the end of the first year. After the second or third year of age, the command of growth is taken over by growth hormone, and this childhood phase is called the growth-hormone-dependent phase of growth. Of course, the growth hormone is not the only hormonal factor involved with growth in this phase; the thyroid hormone, insulin, cortisol, adrenal androgens, and the like also play important roles.3 Some authors (e.g., Karlberg2) claimed to have seen a small spurt in growth velocity at around 2 or 3 years of age and explained this spurt as an expression of the change in command of physiological growth. In puberty, the growth spurt, with its sigmoid-shaped distance curve, is superimposed onto the childhood component and controlled by both the growth hormone and sex steroids. The development of any given phase of growth is influenced by the physiological and pathological variations of the preceding phase.

figure 2-2 The three phases of postnatal growth, according to the ICP model. (Source: Karlberg J. A biologically-oriented mathematical model (ICP) for human growth. Acta Paed Scan. 1989;350:70-94.)

This is to be borne in mind when studying growth curves in children with abnormal growth as a result of, for example, chronic disease or malnutrition.

Changes in Body Fat

Growth of fat tissue follows a completely different pattern, as shown in Figure 2-3, where growth of subcutaneous triceps skinfolds of boys and girls are plotted

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