Puberty refers to the process of physical changes by which a child's body becomes an adult body capable of reproduction. Puberty is initiated by hormone signals from the brain to
the gonads (the ovaries and testes). In response, the gonads produce a variety of hormones that stimulate the growth, function, or
transformation of brain, bones,
muscle, skin, breasts, and
reproductive organs. Growth accelerates
in the first half of puberty and reaches completion by the end. Before puberty, body differences between boys and girls are
almost entirely restricted to the genitalia. During puberty major differences of size, shape,
composition, and function develop in many body structures and systems. The most obvious of these are referred to as
secondary sex characteristics.
In a strict sense, the term puberty (and this article) refers to the bodily changes of sexual maturation rather than the psychosocial and cultural aspects of adolescent development. Adolescence is the period of psychological and social transition between childhood and adulthood. Adolescence largely overlaps the period of puberty but its boundaries are less precisely defined and it
refers as much to the psychosocial and cultural characteristics of development during the teen years as to the physical changes
of puberty.
Differences between male and female puberty
Two of the most significant differences between puberty in girls and puberty in boys are the age at which it begins, and the
major sex steroids involved.
Although there is a wide range of normal ages, on average, girls begin the process of puberty about 1-2 years earlier than
boys, and reach completion in a shorter time.[1] Girls attain adult height and reproductive maturity about 4 years after the first
physical changes of puberty appear. In contrast, boys accelerate more slowly but continue to grow for about 6 years after the
first visible pubertal changes.
The hormone that dominates female development is estradiol, an estrogen. While estradiol promotes growth of breasts and uterus, it is also the
principal hormone driving the pubertal growth spurt and epiphyseal maturation and
closure.[2] Estradiol
levels rise earlier and reach higher levels in women than in men.
In boys, testosterone, an androgen, is the principal
sex steroid. While testosterone produces all the male changes characterized as virilization, a substantial product of testosterone metabolism in males is estradiol, though levels rise
later and more slowly than in girls. The male growth spurt also begins later, accelerates more slowly, and lasts longer before
the epiphyses fuse. Although boys are 2 cm taller than girls before puberty begins, adult men are on average about 13 cm (5.2
inches) taller than women. Most of this sex difference in adult heights is attributable to a later onset of the growth spurt and
a slower progression to completion.[3] a direct result of the later rise and lower adult male levels of estradiol.
Physical changes in boys
Testicular size, function, and fertility
In boys, testicular enlargement is the first physical manifestation of puberty (and is termed gonadarche).[4] Testes in prepubertal boys change little in size from about 1 year of age to the onset of puberty, averaging
about 2–3 cc in volume and about 1.5-2 cm in length. Testicular size continues to increase throughout puberty, reaching maximal
adult size about 6 years later.[5]
While 18-20 cc is reportedly an average adult size, there is wide variation in the normal population.[6]
The testes have two primary functions: to produce hormones and to produce sperm. The Leydig cells produce testosterone (as described below), which in turn produces most of the changes of male sexual maturation and
maintains libido. However, most of the increasing bulk of testicular tissue is spermatogenic
tissue (primarily Sertoli and interstitial
cells). The development of sperm production and fertility in males is not as well
documented. Sperm can be detected in the morning urine of most boys after the first year of pubertal changes (and occasionally
earlier).
Genitalia
A boy's penis grows little from the fourth year of life until puberty. After the penile growth
of early infancy the average childhood penile length is about 7 cm. The prepubertal genitalia are described as stage 1. Within
months after growth of the testes begins, rising levels of testosterone promote growth of the penis and scrotum. This earliest discernible beginning of pubertal growth of the genitalia is referred to as stage
2.[7] The penis continues to grow until about 18 years of
age, reaching an average stretched adult size of about 13 cm.[8]
Although erections and orgasm occur in prepubertal boys,
they become much more common during puberty, accompanied by a markedly increased libido.
Ejaculation becomes possible early in puberty. Prior to this, boys may experience
dry orgasms. Emission of seminal fluid may occur due to masturbation or spontaneously during sleep (commonly termed a wet
dream, and more clinically called a nocturnal emission). The
ability to ejaculate is a fairly early event in puberty compared to the other characteristics.[9] However, in parallel to the irregularity of the first few periods of a girl, for
the first one or two years after a boy's first ejaculation, his seminal fluid may contain few active sperm.
If the foreskin of a boy does not become retractable during childhood, it normally begins to retract during puberty. This
occurs as a result of the increased production of testosterone and other hormones in the body.
Pubic hair in boys
Pubic hair often appears on a boy shortly after the genitalia begin to grow. As in girls,
the first appearance of pubic hair is termed pubarche and the pubic hairs are usually first visible at the dorsal (abdominal)
base of the penis. The first few hairs are described as stage 2. Stage 3 is usually reached within
another 6–12 months, when the hairs are too numerous to count. By stage 4, the pubic hairs densely fill the "pubic triangle." Stage 5 refers to spread of pubic hair to the thighs and
upward towards the navel as part of the developing abdominal
hair.
Body and facial hair
In the months and years following the appearance of pubic hair, other areas of skin which respond to androgens develop heavier
hair (androgenic hair) in roughly the following sequence:
underarm (axillary) hair, perianal hair,
upper lip hair, sideburn (preauricular) hair, periareolar
hair, and the rest of the beard area. Arm, leg, chest,
abdominal, and back hair become heavier more gradually. There is a large range in amount
of body hair among adult men, and significant differences in timing and quantity of hair growth among different ethnic
groups.
Voice change
Under the influence of androgens, the voice box, or larynx,
grows in both genders. This growth is far more prominent in boys, causing the male voice to drop, sometimes abruptly but rarely
"over night", about one octave, because the longer and thicker vocal folds have a lower fundamental frequency. Voices never
"break." A typical 12-year old boy's larynx is larger, even before voice change, than an adult woman's. Occasionally, voice
change is accompanied by unsteadiness of vocalization in the early stages of untrained voices. Most of the voice change happens
during stage 4 of male puberty around the time of peak growth. However, it usually precedes the development of significant facial
hair by several months to years.
Height growth
In boys, growth begins to accelerate about 9 months after the first signs of testicular enlargement and the peak year of the
growth spurt occurs about 2 years after the onset of puberty, reaching a peak velocity of about 8.5–12 cm or 3.5–5 inches per
year. The feet and hands experience their growth spurt first, followed by the limbs, and finally ending in the trunk. Epiphyseal
closure and adult height are reached more slowly, at an average age of about 17.5 years. As in girls, this last growth primarily
involves the spine rather than the limbs.
Male musculature and body shape
By the end of puberty, adult men have heavier bones and nearly twice as
much skeletal muscle. Some of the bone growth (e.g., shoulder width and jaw) is
disproportionately greater, resulting in noticeably different male and female skeletal shapes. The average adult male has about
150% of the lean body mass of an average female, and about 50% of the body fat.
This muscle develops mainly during the later stages of puberty, and muscle growth can continue even after a male is
biologically adult. The peak of the so-called "strength spurt," the rate of muscle growth, is attained about one year after a
male experiences his peak growth rate.
Body odor, skin changes, acne
Rising levels of androgens can change the fatty acid composition of perspiration, resulting in a more "adult" body odor. As in
girls, another androgen effect is increased secretion of oil (sebum) from the skin and the resultant variable amounts of
acne.
Breast development: pubertal gynecomastia
Estradiol is produced from testosterone in male puberty as well as female, and male
breasts often respond to the rising estradiol levels. This is termed gynecomastia. In most boys, the breast development is minimal, similar to what would be termed a "breast
bud" in a girl, but in many boys, breast growth is substantial. It usually occurs after puberty is underway, may increase for a
year or two, and usually diminishes by the end of puberty. It is increased by extra adipose tissue if the boy is overweight.
Although this is a normal part of male puberty for perhaps half of boys, breast development is usually as unwelcome as upper
lip hair in girls, and can be removed surgically if the boy's distress is substantial.
Physical changes in girls
Breast development
The first physical sign of puberty in girls is usually a firm, tender lump under the center of the areola(e) of one or both breasts, occurring on average at about 10.5 years of
age.[10] This is referred to as
thelarche. By the widely used Tanner staging of puberty,
this is stage 2 of breast development (stage 1 is a flat, prepubertal breast). Within six to 12 months, the swelling has clearly
begun in both sides, softened, and can be felt and seen extending beyond the edges of the areolae. This is stage 3 of breast
development. By another 12 months (stage 4), the breasts are approaching mature size and shape, with areolae and papillae forming a secondary mound. In most young women, this mound disappears into the contour of the mature
breast (stage 5), although there is so much variation in sizes and shapes of adult breasts that stages 4 and 5 are not always
separately identifiable.[11]
Pubic hair in girls
Pubic hair is often the second unequivocal change of puberty noticed, usually within a few
months of thelarche.[12] It is referred to
as pubarche and the pubic hairs are usually visible first along the labia. The first few hairs are described as Tanner stage 2.[11] Stage 3 is usually reached within another 6-12 months, when the hairs
are too numerous to count and appear on the pubic mound as well. By stage 4, the pubic hairs
densely fill the "pubic triangle." Stage 5 refers to spread of pubic hair to the thighs and
sometimes as abdominal hair upward towards the navel. In
about 15% of girls, the earliest pubic hair appears before breast development begins.[12]
Vagina, uterus, ovaries
The mucosal surface of the vagina also changes in
response to increasing levels of estrogen, becoming thicker and a duller pink in color (in
contrast to the brighter red of the prepubertal vaginal mucosa).[13] Whitish secretions (physiologic leukorrhea) are a normal effect
of estrogen as well.[10] In the next
2 years following thelarche, the uterus and ovaries increase in
size, and follicles in the ovaries reach larger sizes.[14] The ovaries usually contain small follicular cysts visible by ultrasound.[15][16]
Menstruation and fertility
The first menstrual bleeding is referred to as menarche, and typically occurs about 2 years after thelarche.[12] The average age of menarche in American girls
is about 12.7 years.[12] Menses (menstrual
periods) are not always regular and monthly in the first 2 years after menarche.[17] Ovulation is necessary for
fertility, but may or may not accompany the earliest menses.[18] According to one study, about 80% of menses are anovulatory in the first year
after menarche, and about 50% in the second year, but only 10% of girls were not ovulating after 5 years.[17] However, initiation of ovulation after menarche
is not inevitable, and a high proportion of girls with continued irregularity several years from menarche will continue to have
prolonged irregularity and anovulation, and are at higher risk for reduced fertility.[19] The word nubility is used commonly in
the social sciences to designate achievement of fertility.
Body and facial hair
In the months and years following the appearance of pubic hair, other areas of skin which respond to androgens develop heavier
hair (androgenic hair) in roughly the following sequence:
underarm (axillary) hair, perianal hair,
upper lip hair, sideburn (preauricular) hair, periareolar
hair, and hairs along the linea nigra between from the pubic hair to the umbilicus. The amount of hair in those areas is less than the male amount. There is a large range in
amount of body hair among adult women, and significant differences in timing and quantity of hair growth among different ethnic
groups.
Body shape, fat distribution, and body composition
During this period, also in response to rising levels of estrogen, the lower half of the pelvis widens (providing a larger birth canal).[11] Fat tissue increases to a greater percentage of the body
composition than in males, especially in the typical female distribution of breasts, hips, buttocks, thighs, upper arms, and
pubis. Progressive differences in fat distribution as well as sex differences in local skeletal growth contribute to the typical
female body shape by the end of puberty. At age 10 years, the average girl has 6% more body fat than the average boy, but by the
end of puberty the average difference in nearly 50%.[20]
Body odor, skin changes, and acne
Rising levels of androgens can change the fatty acid
composition of perspiration, resulting in a more "adult" body
odor. This often precedes thelarche and pubarche by 1 or more years. Another androgen effect is increased secretion of oil
(sebum) from the skin. This change increases the susceptibility to acne, a characteristic affliction of puberty greatly variable in its severity.[21]
Variations
Typical puberty is described above, but many children vary with respect to timing of onset, tempo, steadiness of continuation,
and sequence of events.
Timing of onset
Puberty is a process with a gradual onset beginning with changes of neuronal function in the hypothalamus, resulting in rising hormonal signals between
brain and gonads, proceeding to gonadal growth and production of
sex steroids, which in turn induce changes in responsive parts of the body. The definition
of onset, therefore, depends on the perspective (e.g., hormonal versus physical) and purpose (establishing population normal
standards, clinical care of early or late children, or a variety of other social purposes). The most commonly used definition of
onset for both social and medical purposes is the appearance of the first physical changes described in this section of this
article, but it should be understood that these physical changes are the first outward signs of preceding neural, hormonal, and
gonadal function changes that are usually impossible or impractical to detect.
The age at which puberty begins can vary widely between individuals and between populations. Age of puberty is affected by
both genetic factors and by environmental factors such as nutritional state or social circumstances. Timing may also be affected
by environmental factors (exogenous hormones and environmental substances with hormone-like effects) and there is even evidence
that life experiences may play a role as well.
Ethnic/racial differences have been recognized for centuries. For example, the average age of menarche in various populations surveyed in the last several decades has ranged from 12.0 to 18.5 years. The
earliest mean is reported for African-American girls and the oldest for high altitude subsistence populations in Asia. However,
it is clear that much of the higher age averages reflect nutritional limitations more than genetic differences and can change
within a few generations with a substantial change in diet. The median age of menarche for a population may be an index of the
proportion of undernourished girls in the population, and the width of the spread may reflect unevenness of wealth and food
distribution in a population.
Genetic influence
Various studies have found direct genetic effects to account for at least 50% of the variation of timing of puberty in
well-nourished populations. The genetic association of timing is strongest between mothers and daughters. The specific
genes affecting timing are not defined yet. Among the candidates are the androgen and LH receptor genes.
Environmental factors
If genetic factors account for half of the variation of pubertal timing, environment factors are clearly important as well.
One of the earliest observed environmental effects is that puberty occurs later in children raised at higher altitudes. The most
important of the environmental influences is clearly nutrition, but a number of others have been identified, all which affect
timing of female puberty and menarche more clearly than male puberty.
Nutritional influence
Nutritional factors are the strongest and most obvious environmental factors affecting
timing of puberty. Girls are especially sensitive to nutritional regulation because they must contribute all of the nutritional
support to a growing fetus. Surplus calories (beyond growth and activity requirements) are
reflected in the amount of body fat, which signals to the brain the availability of
resources for initiation of puberty and fertility.
Much evidence suggests that for most of the last few centuries, nutritional differences accounted for majority of variation of
pubertal timing in different populations, and even among social classes in the same population. Recent worldwide increased
consumption of animal protein, other changes in nutrition, and increases in childhood fatness have resulted in falling ages of
puberty, mainly in those populations with the higher previous ages. In many populations the amount of variation attributable to
nutrition is shrinking.
Although available dietary energy (simple calories) is the most important dietary influence on timing of puberty, quality of
the diet plays a role as well. Lower protein intakes and higher plant fiber intakes, as occur with typical vegetarian diets, are
associated with later onset and slower progression of female puberty.
Studies have shown that calcium deficiency is a cause of late puberty, irregular and painful, cramping during menstruation
with excessive blood loss, and lowered immune response to infections in young girls. This could be from a deficient diet or lack
of vitamin D from too little sun exposure. This lack of calcium could predispose them to osteoporosis later in life.
Physical activity and exercise
The average level of daily physical activity has also been shown to affect timing of puberty, especially female. A high level
of exercise, whether for athletic or body image purposes, or for daily subsistence, reduces energy calories available for
reproduction and slows puberty. The exercise effect is often amplified by a lower body fat mass.
Physical illness
Many chronic diseases can delay puberty in both boys and girls. Those that involve chronic inflammation or interfere with
nutrition have the strongest effect. In the western world, inflammatory bowel
disease and tuberculosis have been notorious for such an effect in the last century,
while in areas of the underdeveloped world, chronic parasite infections are widespread.
Environmental chemicals and hormones
There is theoretical concern, and animal evidence, that environmental hormones and chemicals may affect aspects of prenatal or postnatal sexual development in humans. Large amounts of
incompletely metabolized estrogens and progestagens from pharmaceutical products are
excreted into the sewage systems of large cities, and are sometimes detectable in the environment. Sex steroids are sometimes used in cattle farming but have been banned in chicken meat production for 40
years. Although agricultural laws regulate use to minimize accidental human consumption, the rules are largely self-enforced in
the United States. Significant exposure of a child to hormones or other substances that activate estrogen or androgen
receptors could produce some or all of the changes of puberty.
Harder to detect as an influence on puberty are the more diffusely distributed environmental chemicals like PCBs
(polychlorinated biphenyl), which can bind and trigger estrogen receptors.
More obvious degrees of partial puberty from direct exposure of young children to small but significant amounts of
pharmaceutical sex steroids from exposure at home may be detected during medical evaluation for precocious puberty, but mild effects and the other potential exposures outlined above would not.
Stress and social factors
Some of the least understood environmental influences on timing of puberty are social and psychological. In comparison with
the effects of genetics, nutrition, and general health, social influences are small, shifting timing by a few months rather than
years. Mechanisms of these social effects are unknown, though a variety of physiological processes, including pheromones, have been suggested based on animal research.
The most important part of a child's psychosocial environment is the family, and most of the social influence research has
investigated features of family structure and function in relation to earlier or later female puberty. Most of the studies have
reported that menarche may occur a few months earlier in girls in high-stress households, whose fathers are absent during their
early childhood, who have a stepfather in the home, who are subjected to prolonged sexual
abuse in childhood, or who are adopted from a developing country at a
young age. Conversely, menarche may be slightly later when a girl grows up in a large family with a biological father
present.
More extreme degrees of environmental stress, such as wartime refugee status with threat to physical survival, have been found
to be associated with delay of maturation, an effect that may be compounded by dietary inadequacy.
Most of these reported social effects are small and our understanding is incomplete. Most of these "effects" are statistical
associations revealed by epidemiologic surveys. Statistical associations are not
necessarily causal, and a variety of covariables and alternative explanations can be imagined. Effects of such small size can
never be confirmed or refuted for any individual child. Furthermore, interpretations of the data are politically controversial
because of the ease with which this type of research can be used for political advocacy. Accusations of bias based on political
agenda sometimes accompany scientific criticism.
Another limitation of the social research is that nearly all of it has concerned girls, partly because female puberty requires
greater physiologic resources and partly because it involves a unique event (menarche) that makes survey research into female
puberty much simpler than male. More detail is provided in the menarche article.
Average timing for American children
Some of the most complete reference data are available for American children and are included here. Average age for first
signs of breast development in girls is about 10.5 years. Average age for first signs of testicular enlargement in boys is 11.5
years. See Tables below for approximate average ages and ranges for other milestones of physical development of North American
children.
Duration of puberty (time from onset to completion) varies less between children than does the age of onset. Duration of
puberty in girls from onset of breast development to cessation of growth is roughly 5 years. Duration of puberty in boys from
first testicular enlargement to cessation of growth is about 6 years.
Table 1 provides 3rd, 50th, and 97th percentiles for attainment of selected stages
by American girls, based on NCHS data collected in the 1970s and
reported in 1985 (Tanner et al., 1985). In these tables, B, PH, and G refer to the Tanner stages of physical puberty: B is breast, PH is pubic hair, and G is genitalia
(penis and testes). B1, PH1, and G1 are the prepubertal stages of each of these, while B2, PH2, and
G2 are the earliest signs of puberty. B5, PH5, and G5 are adult stages at the end of puberty. The
Tanner stage article contains links to fuller explanations of the specific stages. All
three tables below express ages as years and months (y and m).
Table 1: Ages of attainment of pubertal stages of American girls
| Stages |
3rd percentile |
50th percentile |
97th percentile |
| B2 |
8y 10 m |
10y 11 m |
13y 0 m |
| B3 |
9y 10 m |
11y 11 m |
14y 0 m |
| B4 |
10y 6 m |
12y 11 m |
14y 5 m |
| PH2 |
9y 0 m |
11y 3 m |
13y 6 m |
| PH3 |
9y 8 m |
11y 11 m |
14y 3 m |
| PH4 |
10y 5 m |
12y 7 m |
14y 7 m |
| Menarche |
10y 10 m |
12y 9 m |
14y 7 m |
| Peak height velocity |
9y 0 m |
11y 6 m |
14y 0 m |
However, a later survey performed in the mid-1990s by a group of American primary pediatric practices with slightly different
methods reported both a mildly earlier average onset, greater range, and more importantly, a significant difference between white
and African-American girls in the early stages (Table 2) (Herman-Giddens et al.).
Table 2a: 1997 survey of white American girls
| Stages |
3rd percentile |
50th percentile |
97th percentile |
| B2 |
6y 5 m |
10y 0 m |
13y 7 m |
| B3 |
8y 7 m |
11y 4 m |
14y 1 m |
| B4 |
10y 4 m |
12y 9 m |
15y 3 m |
| B5 |
11y 4 m |
14y 6 m |
17y 9 m |
| PH2 |
7y 2 m |
10y 5 m |
13y 8 m |
| PH3 |
8y 8 m |
11y 5 m |
14y 2 m |
| PH4 |
10y 5 m |
14y 7 m |
14y 7 m |
| PH5 |
10y 10 m |
12y 2 m |
16y 8 m |
| Menarche |
10y 6 m |
12y 10 m |
15y 3 m |
| Peak height velocity |
10y 0 m |
12y 6 m |
14y 0 m |
Table 2b: Recent survey of African-American girls
| Stages |
3rd percentile |
50th percentile |
97th percentile |
| B2 |
5y 0 m |
8y 11 m |
12y 10 m |
| B3 |
7y 7 m |
10y 2 m |
12y 11 m |
| PH2 |
4y 9 m |
8y 9 |
