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Physical Growth of Infants and Children

By

Evan G. Graber

, DO, Sydney Kimmel Medical College

Last full review/revision Feb 2019| Content last modified Feb 2019
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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). At puberty, a 2nd growth spurt occurs, affecting boys and girls slightly differently.

From birth until age 2 years, it is recommended that all growth parameters be charted using standard growth charts from the WHO. After age 2, growth parameters are charted using growth charts from the CDC (1).

Reference

Length

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.

Before 12 months, height velocity varies and is due in part to perinatal factors (eg, prematurity). After 12 months, height is mostly genetically determined, and height velocity stays nearly constant until puberty; a child’s height relative to peers tends to remain the same.

Some small-for-gestational-age infants tend to be shorter throughout life than infants whose size is appropriate for their gestational age. Boys and girls show little difference in height and growth rate during infancy and childhood.

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

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 (see table Changes in Prevalence of Obesity According to NHANES) has involved markedly greater weight gain, even among very young children. In general, boys are heavier and taller than girls when growth is complete because boys have a longer prepubertal growth period, increased peak velocity during the pubertal growth spurt, and a longer adolescent growth spurt.

Table
icon

Changes in Prevalence of Obesity According to NHANES

Age Group

1976–1980

2003–2004

2007–2008

2009–2010

2011-2012

2013–2014

2015–2016

2–5 years

5%

13.9%

10.1%

12.1%

8.4%

9.4%

13.9%

6–11 years

6.5%

18.8%

19.6%

18.0%

17.7%

17.4%

18.4%

12–19 years

5%

17.4%

18.1%

18.4%

20.5%

20.6%

20.6%

20–74 years

15%

32.9%

33.7%

35.7%

34.9%

37.7%

39.6%

Hales CM, Fryar CD, Carroll MD, et al: Trends in obesity and severe obesity prevalence in US youth and adults by sex and age, 2007-2008 to 2015-2016. JAMA 319 (16):1723–1725, 2018. doi:10.1001/jama.2018.3060.

NHANES = National Health and Nutrition Examination Surveys.

Head Circumference

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

Body composition (proportions of body fat and water) changes and affects drug volume of distribution. 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

Tooth eruption is variable (see Table: Tooth Eruption Times), primarily because of genetic factors. On average, normal infants should have 6 teeth by 12 months, 12 teeth by 18 months, 16 teeth by 2 years, and all teeth (20) by 2½ years; deciduous teeth are replaced by permanent teeth between the ages of 5 years and 13 years. Eruption of deciduous teeth is similar in both sexes; permanent teeth tend to appear earlier in girls. Tooth eruption may be delayed by familial patterns or by conditions such as rickets, hypopituitarism, hypothyroidism, or Down syndrome. Supernumerary teeth and congenital absence of teeth are probably normal variants.

Identifying the teeth

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

Identifying the teeth
Table
icon

Tooth Eruption Times

Teeth

Number

Age at Eruption*

Deciduous (20 total)

Lower central incisors

2

5–9 months

Upper central incisors

2

8–12 months

Upper lateral incisors

2

10–12 months

Lower lateral incisors

2

12–15 months

1st molars

4

10–16 months

Canines

4

16–20 months

2nd molars

4

20–30 months

Permanent (32 total)

1st molars

4

5–7 years

Incisors

8

6–8 years

Bicuspids

8

9–12 years

Canines

4

10–13 years

2nd molars

4

11–13 years

3rd molars

4

17–25 years

*Varies greatly.

Molars are numbered from the front to the back of the mouth (see Figure: Identifying the teeth).

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