Puberty--the biological transition to adult reproductive appearance and function--takes place through a defined process of steps. Doctors measure the stage of puberty using a standard called the Tanner stages. In each stage there are certain hormonal and physical characteristics changes that occur, ranging from the period before puberty to biological adulthood.
In Tanner Stage 1, the child is pre-pubertal. The levels of reproductive hormones are low, and there are no signs of development of the secondary sex characteristics such as breasts for girls, penis growth for boys, and appearance of pubic hair for both. Children are expected to grow about 5cm to 6cm in height per year, below the rate of the growth spurts typical in later stages of puberty.
Tanner Stage 2 is the first stage of actual puberty. At this stage, the hormonal axis has matured enough to start causing visible signs of puberty. In girls, breast buds develop. In boys, the scrotum -- the sac that holds the testes -- and testes grow and the scrotum might look red and less smooth. You can expect an increased rate of growth in girls of 7cm to 8 cm per year, while boys continue to grow at the pre-pubertal rate of 5cm to 6cm per year. Sparse and relatively light and straight pubic hairs appear in boys and girls.
Girls’ breasts continue to grow during this stage, but the areola is still flush with the breast. For boys, the penis starts to grow longer, and the testes keep growing. Boys can expect to grow about 7cm to 8cm taller per year, and girls reach their peak growth rate of about 8cm per year. For both sexes, pubic hair darkens and grows coarser and curlier.
Stage 4 is the final stage of pubertal changes. In girls, the areola of the breast now forms a distinct mound above the breast itself. Boys’ penises grow thicker, and the testes and scrotum are larger than in Tanner Stage 3. The skin of the scrotum also gets darker. Boys reach their peak growth in height of about 10cm per year; girls’ growth is already slowing to about 7cm per year. The pubic hair looks like adult pubic hair but does not cover as much area.
At Tanner Stage 5, puberty is really over and the former child is now a biological adult. Breasts and penises look fully mature, as does pubic hair. Boys reach their maximum height when they are about 17 and girls stop growing taller around 16.