Which hormones are responsible for the constant growth rate throughout infancy and childhood?

Topic Resources

Physical growth includes attainment of full height and appropriate weight and an increase in size of all organs (except lymphatic tissue, which decreases in size). Growth from birth to adolescence occurs in 2 distinct phases:

• Phase 1 (from birth to about age 1 to 2 years): This phase is one of rapid growth, although the rate of growth decreases over that period.

• Phase 2 (from about 2 years to the onset of puberty): In this phase, growth occurs in relatively constant annual increments.

Puberty is the process of physical maturation from child to adult. Adolescence defines an age group; puberty occurs during adolescence (see Physical Growth and Sexual Maturation of Adolescents Physical Growth and Sexual Maturation of Adolescents During adolescence (usually considered age 10 to the late teens or early 20s), boys and girls reach adult height and weight and undergo puberty. For boys, see Sexual Differentiation, Adrenarche... read more ). At puberty, a 2nd growth spurt occurs, affecting boys Sexual Differentiation, Adrenarche, and Puberty Male sexual development and hormonal function depend on a complex feedback circuit involving the hypothalamus-pituitary-testes modulated by the central nervous system. Male sexual dysfunction... read more and girls Puberty Hormonal interaction between the hypothalamus, anterior pituitary gland, and ovaries regulates the female reproductive system. The hypothalamus secretes a small peptide, gonadotropin-releasing... read more slightly differently.

Length is measured in children too young to stand; height is measured once the child can stand. In general, length in normal-term infants increases about 30% by 5 months and > 50% by 12 months; infants grow about 25 cm during the first year, and height at 5 years is about double the birth length. In most boys, half the adult height is attained by about age 2; in most girls, height at 19 months is about half the adult height.

Rate of change in height (height velocity) is a more sensitive measure of growth than time-specific height measures. In general, healthy term infants and children grow about 2.5 cm/month between birth and 6 months, 1.3 cm/month from 7 to 12 months, and about 7.6 cm/year between 12 months and 10 years.

Extremities grow faster than the trunk, leading to a gradual change in relative proportions; the crown-to-pubis/pubis-to-heel ratio is 1.7 at birth, 1.5 at 12 months, 1.2 at 5 years, and 1.0 after 7 years.

Weight follows a similar pattern. Normal-term neonates generally lose 5 to 8% of birth weight in the days after delivery but regain their birth weight within 2 weeks. They then gain 14 to 28 g/day until 3 months, then 4000 g between 3 and 12 months, doubling their birth weight by 5 months, tripling it by 12 months, and almost quadrupling it by 2 years. Between age 2 years and puberty, weight increases 2 kg/year. The recent epidemic of childhood obesity Children Obesity is excess body weight, defined as a body mass index (BMI) of ≥ 30 kg/m2. Complications include cardiovascular disorders (particularly in people with excess abdominal fat)... read more (see table Changes in Prevalence of Obesity According to NHANES Changes in Prevalence of Obesity According to NHANES

Changes in Prevalence of Obesity According to NHANES

Age Group

1976–1980

2003–2004

2007–2008

2009–2010

2011-2012

2013–2014

2015–2016

Head circumference reflects brain size and is routinely measured up to 36 months. At birth, the brain is 25% of adult size, and head circumference averages 35 cm. Head circumference increases an average 1 cm/month during the first year; growth is more rapid in the first 8 months, and by 12 months, the brain has completed half its postnatal growth and is 75% of adult size. Head circumference increases 3.5 cm over the next 2 years; the brain is 80% of adult size by age 3 years and 90% by age 7 years.

Body composition (proportions of body fat and water) changes and affects drug volume of distribution Distribution Pharmacokinetics refers to the processes of drug absorption, distribution, metabolism, and elimination. There are important age-related variations in pharmacokinetics. Absorption from the gastrointestinal... read more . Proportion of fat increases rapidly from 13% at birth to 20 to 25% by 12 months, accounting for the chubby appearance of most infants. Subsequently, a slow fall occurs until preadolescence, when body fat returns to about 13%. There is a slow rise again until the onset of puberty, when body fat may again fall, especially in boys. After puberty, the percentage generally stays stable in girls, whereas in boys there tends to be a slight decline.

Body water measured as a percentage of body weight is 70% at birth, dropping to 61% at 12 months (about equal to the adult percentage). This change is fundamentally due to a decrease in extracellular fluid from 45% to 28% of body weight. Intracellular fluid stays relatively constant. After age 12 months, there is a slow and variable fall in extracellular fluid to adult levels of about 20% and a rise in intracellular fluid to adult levels of about 40%. The relatively larger amount of body water, its high turnover rate, and the comparatively high surface losses (due to a proportionately large surface area) make infants more susceptible to fluid deprivation than older children and adults.

Tooth eruption is variable (see table Tooth Eruption Times Tooth Eruption Times

Tooth Eruption Times

Teeth

Number

Age at Eruption*

Identifying the teeth

The numbering system shown is the one most commonly used in the US.

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