A child's height is primarily determined by the length of his or her bones, and thus children grow taller because their bones grow longer. The bones grow longer because they contain growth plates -- thin layers of cartilage found near the ends of the bones. Cartilage is a firm, resilient material found in your ears and nose, as well as lining the ends of your bones at the joints. Children also have cartilage farther inside the bones, forming the growth plates. Within the growth plates, cells divide and enlarge, producing more cartilage, which is subsequently converted into bone, and this process causes the bones to elongate.
Grwoth is rapid early in life, then slows
Prior to birth, your child grew at an enormous rate -- starting as a microscopic fertilized egg and then growing about 20 inches and about 6 to 9 pounds in the nine months of pregnancy. If that growth rate were to continue after birth, the child would reach adult size in two or three years! However, the growth rate slows down with age. In the first year after birth, an average child grows about 10 inches in length, and in the second year, a child usually grows about 5 inches. By the time he or she is seven years old, the average child is only growing about 2 inches per year.
The adolescent growth spurt
This progressive decline in the growth rate is interrupted by the adolescent growth spurt (also known as the pubertal growth spurt). Sometime after puberty starts, the child's growth speeds up for a couple of years. Most girls start puberty sometime between 8 and 13 years of age. The first visible signs are breast development and pubic hair. In girls, the growth spurt occurs early in puberty. Growth starts to speed up even as the first signs of puberty appear. In boys, puberty starts a bit later, usually between age 9 and 14. The first visible signs of puberty are pubic hair and growth of the testes (testicles) and penis. Unlike girls, the adolescent growth spurt in boys does not start early in puberty; it occurs a year or two later. In both boys and girls, the growth rate can double during the adolescent growth spurt, increasing from 2 inches per year to 3 or 4 inches per year.
The end of growth
The adolescent growth spurt does not last very long. After one to two years of rapid growth, the growth rate starts to slow. By the time a girl has her first menstrual period (typically two years after the start of puberty), the growth rate has usually fallen to approximately 2 inches per year. At this time, the average girl has about 3 inches of growth remaining, but some grow little more than another inch while others grow more than 4. Boys' growth also slows as puberty comes to completion.
Growth does not stop abruptly. The growth rate decreases gradually, dropping from about 3 or 4 inches per year at the peak of the spurt to about 2 inches the next year, then 1 inch per year, then 1/2 inch. By the time the average girl reaches her 15th birthday and the average boy reaches his 17th, they have less than an inch of growth left.
Remember that all these numbers are just approximations, and, as we'll discuss in the next section, the age at which growth tapers off varies widely among individual children. Eventually, however, growth in height does stop completely. In fact, the growth plates disappear from the young man or woman's bones, a process called epiphyseal fusion. Once the growth plates are gone, no further increase in height occurs.
The rate of maturation
Different children mature at different rates. Dr James Tanner, a pioneer in the field of growth research, compared the rate of maturation to a musical tempo. This tempo can affect the child's growth rate, how old he or she looks, when he or she loses baby teeth, and when the child starts puberty.
Some children mature at a slow tempo. These kids often look young for their age and are shorter than average. They go into puberty late and so they have their growth spurt later than most of their peers. When this occurs, the child with the slow pace of maturation feels particularly short because his friends are all shooting up in height. In a year or two, however, the slowly maturing child will begin his growth spurt. By that time, his friends' growth will be slowing down and so the child will start to catch up to his peers. Usually, he will continue to grow after his friends have stopped growing. Some boys with a slow tempo of maturation will still be growing after graduating from high school. Slowly maturing children can end up short, average, or tall as adults.
Other children mature at a rapid tempo. They are often tall in childhood. They also tend to go into puberty early. The resulting early growth spurt can make them feel very tall compared with their friends. However, these rapidly maturing kids tend to stop growing sooner, at which time their peers tend to catch up. As with slowly maturing kids, the adult height can be tall, average, or even short.
What controls growth?
Why does one child grow more rapidly than another? Why does one child end up taller than another? Three principal factors regulate a child's growth:
When a child is not growing at a normal rate, one or more of these three factors are probably responsible.
Key points to keep in mind
Children grow taller because of growth plates present within their bones. Growth is very rapid in early life but decreases with age. This decline is briefly interrupted by the adolescent growth spurt, after which the growth rate again decreases. Growth finally stops in late adolescence, and the growth plates disappear. The tempo of growth and physical maturation varies among different children, with some maturing rapidly and others slowly.
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