Physical Growth of Infants and Children

Length is measured with a supine stadiometer in an infant. The infant is laid on the stadiometer. The infant's head is held so that the crown is flat against the head plate. Gently, the infant's legs are straightened and the knees are pressed down. Then the foot plate is moved until it touches the infant's heels. Three measurements should be taken and averaged to determine an accurate length measurement.

Height is measured with a standing stadiometer once a child can stand. The child stands against the stadiometer. The child's feet should be flat on the floor and the heels flat against the wall. The child's head should be positioned so that the eyes are parallel to the floor. Then the head plate of the stadiometer is brought down to touch the crown of the head. Again, three measurements should be taken and averaged to determine an accurate height measurement.

In general, length in full-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. Most boys reach half their adult height by about age 2 years; most girls reach half their adult height at about age 19 months.

Rate of change in height (height velocity) is a more sensitive measure of growth than time-specific height measurements. 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 of age, height velocity varies and is due in part to perinatal factors (eg, prematurity Preterm Infants An infant born before 37 weeks gestation is considered preterm. Prematurity is defined by the gestational age at which infants are born. Previously, any infant weighing < 2.5 kg was termed... read more ). 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.

In general, boys weigh more and are 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.

Some small-for-gestational-age infants Small-for-Gestational-Age (SGA) Infant Infants whose weight is < the 10th percentile for gestational age are classified as small for gestational age. Complications include perinatal asphyxia, meconium aspiration, polycythemia... read more 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 follows a similar pattern. Full-term neonates generally lose 5 to 8% of birth weight in the first few 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 approximately 2 kg/year.

The prevalence 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 ) began to increase significantly in the United States in the 1980s, with markedly greater weight gain, even among very young children. The prevalence of childhood and adolescent obesity remains high today (1 Weight reference 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... read more ).

The CDC has released extended BMI-for-age growth charts for boys and girls with a very high BMI value.

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 ), 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 20 teeth 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. Symptoms associated with tooth eruption are called teething Teething Teething is the process in infants of tooth eruption through the gums. A child's first tooth usually erupts by 6 months of age, and a complete set of 20 deciduous teeth usually develops by 2½... read more .

Tooth eruption may be delayed by familial patterns or by conditions such as rickets Hypophosphatemic Rickets Hypophosphatemic rickets is a genetic disorder characterized by hypophosphatemia, defective intestinal absorption of calcium, and rickets or osteomalacia unresponsive to vitamin D. It is usually... read more , hypopituitarism Growth Hormone Deficiency in Children Growth hormone deficiency is the most common pituitary hormone deficiency in children and can be isolated or accompanied by deficiency of other pituitary hormones. Growth hormone deficiency... read more , hypothyroidism Hypothyroidism in Infants and Children Hypothyroidism is thyroid hormone deficiency. Symptoms in infants include poor feeding and growth failure; symptoms in older children and adolescents are similar to those of adults but also... read more , or Down syndrome Down Syndrome (Trisomy 21) Down syndrome is an anomaly of chromosome 21 that can cause intellectual disability, microcephaly, short stature, and characteristic facies. Diagnosis is suggested by physical anomalies and... read more . Supernumerary teeth and congenital absence of teeth are probably normal variants.

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