Physical Growth of Infants and Children
Physical growth refers to an increase in body size (length or height and weight) and in the size of organs. From birth to about age 1 or 2 years, children grow rapidly. After this time, growth slows. As growth slows, children need fewer calories and parents may notice a decrease in appetite. Two-year-old children can have very erratic eating habits that sometimes make parents anxious. Some children may seem to eat virtually nothing yet continue to grow and thrive. Actually, they usually eat little one day and then make up for it by eating more the next day.
During the preschool and school years, growth in height and weight is steady. Children tend to grow a similar amount each year until the next major growth spurt occurs in early adolescence.
Different organs grow at different rates. For example, the reproductive system has a brief growth spurt just after birth, then changes very little until just before sexual maturation (puberty). In contrast, the brain grows almost exclusively during the early years of life. The kidneys function at the adult level by the end of the first year.
Children who are beginning to walk have an endearing physique, with the belly sticking forward and the back curved. They may also appear to be quite bow-legged. By 3 years of age, muscle tone increases and the proportion of body fat decreases, so the body begins to look leaner and more muscular. Most children are physically able to control their bowels and bladder at this time.
Doctors report how children are growing in relation to other children their age and monitor the children's weight gain compared to their height. From birth until 2 years of age, doctors record all growth parameters in a chart by using standard growth charts from the World Health Organization (WHO). After age 2, doctors record growth parameters by using growth charts from the Centers for Disease Control and Prevention (CDC).
(See also Health Supervision of the Well Child and Failure to Thrive.)
Length and Height
Length in children who are too young to stand is measured while children lie on their back on a suitable device, such as a measuring table (called a stadiometer). Height in children who can stand is measured using a vertical measuring scale. In general, length in normal-term infants increases about 30% by age 5 months and more than 50% by age 12 months. Infants typically grow about 10 inches (25 centimeters) during the first year, and height at 5 years is about double the birth length. In boys, half the adult height is attained by about age 2. In girls, height at 19 months is about half the adult height.
Weight
Normal-term newborns typically lose 5 to 8% of their birth weight during the first few days of life. They regain this weight by the end of the first 2 weeks. After this period of time, newborns typically gain about 1 ounce per day during the first 2 months, and 1 pound per month after that. This weight gain typically results in a doubling of birth weight by age 5 months and a tripling by 1 year. In recent years, more children have developed obesity. Some children become obese at an early age.
Weight and Length Charts for Infants from Birth to 24 Months of Age
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During the first year of life, an infant's weight and length are charted at each doctor's visit to make sure that growth is proceeding at a steady rate. Percentiles are a way of comparing infants of the same age. For an infant at the 10th percentile for weight, 10% of infants weigh less and 90% weigh more. For an infant at the 90th percentile, 90% of infants weigh less and 10% weigh more. For an infant at the 50th percentile, 50% of infants weigh less and 50% weigh more. Of more significance than the actual percentile is any significant change in percentile between doctor's visits. (Adapted from WHO Child Growth Standards. Available at www.cdc.gov/growthcharts.) 
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Height and Weight Charts for Boys and Girls 2 to 10 Years of Age
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Adapted from The National Center for Health Statistics in collaboration with the National Center for Chronic Disease Prevention and Health Promotion (2000). Available at www.cdc.gov/growthcharts. 
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Head Circumference
Head circumference is measurement around the largest area of a child's head. Doctors place the tape measure above the eyebrows and ears and around the back of the head. This measurement is important because the size of the head reflects the size of the brain, and this measurement lets doctors know whether the child's brain is growing at a normal rate. Head circumference is routinely measured until children are 3 years old.
At birth, the brain is 25% of its future adult size, and head circumference is about 14 inches (about 35 centimeters). By 1 year of age, the brain is 75% of its adult size. By 3 years of age, the brain is 80% of its adult size. By 7 years of age, the brain is 90% of its adult size.
Teeth
The timing of tooth eruption varies, mainly for hereditary reasons. However, tooth eruption may also be delayed by disorders such as rickets, hypopituitarism, hypothyroidism, or Down syndrome.
Lower front teeth usually begin to appear by the age of 5 to 9 months. Upper front teeth usually begin to appear by 8 to 12 months. On average, infants have 6 teeth by age 12 months, 12 teeth by 18 months, 16 teeth by 2 years, and all 20 of their baby (deciduous) teeth by 2½ years. Baby teeth are replaced by permanent (adult) teeth between the ages of 5 years and 13 years. Permanent teeth tend to appear earlier in girls.
Tooth Eruption Times
More Information
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Growth charts from the World Health Organization (WHO)
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Growth charts from the Centers for Disease Control and Prevention (CDC)
