Puberty (cont.)

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What determines when puberty begins?

The timing of the onset of puberty is not completely understood and is likely determined by a number of factors. One theory proposes that reaching a critical weight or body composition may play a role in the onset of puberty. It has been proposed that the increase in childhood obesity may be related to the overall earlier onset of puberty in the general population in recent years.

Leptin, a hormone produced by fat cells (adipocytes) in the body, has been suggested as a possible mediator of the timing of puberty. In research studies, animals deficient in leptin did not undergo puberty, but puberty began when leptin was administered to the animals. Further, girls with higher concentrations of the hormone leptin are known to have an increased percentage of body fat and an earlier onset of puberty than girls with lower levels of leptin. The concentration of leptin in the blood is known to increase just before puberty in both boys and girls.

Leptin likely is one of multiple influences on the hypothalamus, an area of the brain that releases a hormone known as gonadotropin-releasing hormone (GnRH), which in turn signals the pituitary gland to release leutinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH secretion by the pituitary is responsible for sexual development.

A gene has been identified that appears to be critical for the normal development of puberty. The gene, known as GPR54, encodes a protein that appears to have an effect on the secretion of GnRH by the hypothalamus. Individuals who do not have a functioning copy of this gene are not able to enter puberty normally.

What are the physical changes of puberty in girls and boys?

The changes that happen during the process of puberty have a typical pattern in both boys and girls, with a generally predictable sequence of events. In most girls, the first sign of puberty is the beginning of breast development, which occurs at an average age of approximately 11 years. In girls, the growth of pubic hair typically begins next, followed by the growth of hair in the armpits. A minority of girls, however, begin to develop pubic hair prior to breast development. The onset of menstruation (having periods) usually happens later than the other physical changes and usually occurs around two and a half years after the onset of puberty.

A regular pattern of ovulation, corresponding to achievement of fertility, usually develops rapidly once a girl begins having menstrual periods. However, girls who have a later onset of menstruation (after age 13) tend to have lower rates of regular ovulation in the years following the onset of menstruation. Studies have shown that one-half of adolescent girls who first begin to menstruate after age 13 will not ovulate regularly over the next four and a half years.

In boys, an increase in the size of the testicles is the first change observed at the onset of puberty. Enlargement of the testicles begins at an approximate average age of 11 and a half years in boys and lasts for about six months. After enlargement of the testicles, the penis also increases in size. Enlargement of the testicles and penis almost always occurs before the development of pubic hair. The next stage is the growth of pubic hair and hair in the armpits. Next, the voice becomes deeper and muscles increase in size. The last step is usually the development of facial hair.

Fertility is achieved in males near the onset of puberty, when a surge in testosterone triggers the production of sperm.

The sequence of changes in puberty has been characterized by physicians and is referred to as sexual maturity rating (SMR) or Tanner stages, named after a physician who published a description of the sequence of physical changes in puberty in 1969. Tanner stages are determined by the development of the secondary sex characteristics and encompass changes in the size and appearance of the external genitalia, the development of pubic hair, and breast development in girls. Tanner stages allow doctors to classify the extent of development of sex characteristics into five distinct steps ranging from stage 1 (prepubertal) to stage 5 (mature adult type).


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