Puberty is the process of physical changes through which achild'sbody matures into anadult body capable ofsexual reproduction. It is initiated byhormonal signals from thebrain to thegonads: theovaries in a female, thetesticles in a male. In response to the signals, the gonads produce hormones that stimulatelibido and the growth, function, and transformation of the brain,bones,muscle,blood,skin,hair,breasts, andsex organs.Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed. Before puberty, the external sex organs, known asprimary sexual characteristics, are sex characteristics that distinguish males and females. Puberty leads tosexual dimorphism through the development of thesecondary sex characteristics, which further distinguish the sexes.
On average, females begin puberty at age 10½ and complete puberty at ages 15-17; males begin at ages 11½-12 and complete puberty at ages 16-17.[1][2][3] The major landmark of puberty for females ismenarche, the onset of menstruation, which occurs on average around age 12½.[2] For males, firstejaculation,spermarche, occurs on average at age 13.[4] In the 21st century, the average age at which children, especially females, reach specific markers of puberty is lower compared to the 19th century, when it was 15 for females and 17 for males (with age at first periods for females and voice-breaks for males being used as examples).[5] This can be due to any number of factors, including improved nutrition resulting in rapid body growth, increased weight and fat deposition,[6] or exposure toendocrine disruptors such asxenoestrogens, which can at times be due to food consumption or other environmental factors.[7][8] However, more modern archeological research suggests that the rate of puberty as it occurs now is the intended way. Growth spurts began at around 10-12, but markers of later stages of puberty such as menarche had delays that correlated with severe environmental conditions such as poverty, poor nutrition, air and pollution.[9][10][11] Puberty that starts earlier than usual is known asprecocious puberty, and puberty which starts later than usual is known asdelayed puberty.
Notable among themorphologic changes in size, shape, composition, and functioning of the pubertal body, is the development of secondary sex characteristics, the "filling in" of the child's body; from girl to woman, from boy to man. Derived from theLatinpuberatum (age of maturity), the wordpuberty describes the physical changes to sexual maturation, not thepsychosocial and cultural maturation denoted by the termadolescent development inWestern culture, whereinadolescence is the period of mental transition from childhood toadulthood, which overlaps much of the body's period of puberty.[12]
Differences between male and female puberty
Approximate outline of development periods fromchildhood to early adulthood. Puberty is marked in green at right.
Two of the most significant differences between puberty in females and puberty in males are the age at which it begins, and the majorsex steroids involved, theandrogens and theestrogens.
Although there is a wide range of normal ages, females typically begin the process of puberty around age 10½; males at ages 11½—12. Puberty generally ends between 15—17 for females and 16–17 for males.[1][2][3] Females attain reproductive maturity about four years after the first physical changes of puberty appear.[13] In contrast, males accelerate more slowly but continue to grow for about six years after the first visible pubertal changes.[14]
For males, theandrogentestosterone is the principalsex hormone; while testosterone is produced, all males' changes are characterized asvirilization. A substantial product of testosterone metabolism in males is theestrogenestradiol. The conversion of testosterone to estradiol depends on the amount of body fat and estradiol levels in males are typically much lower than in females. The male "growth spurt" also begins later, accelerates more slowly, and lasts longer before theepiphyses fuse. Although males are on average 2 centimetres (0.8 in) shorter than females before puberty begins, adult men are on average about 13 centimetres (5.1 in)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, a direct result of the later rise and lower adult male levels of estradiol.[15]
The hormonal maturation of females is considerably more complicated than in males. The mainsteroid hormones, testosterone, estradiol, andprogesterone as well asprolactin play important physiological functions in puberty. The production ofgonadal steroids in females starts with production of testosterone, which is typically quickly converted to estradiol inside the ovaries. However the rate of conversion from testosterone to estradiol (driven by FSH/LH balance) during early puberty is highly individual, resulting in very diverse development patterns ofsecondary sexual characteristics. Production of progesterone in the ovaries begins with the development of ovulatory cycles in females (during the lutheal phase of the cycle), before puberty low levels of progesterone are produced in the adrenal glands of both males and females. Estradiol levels rise earlier and reach higher levels in women than in men. While estradiol promotes growth of the breasts anduterus, it is also the principal hormone driving the pubertal growth spurt and epiphyseal maturation and closure.[16]
Puberty onset
Puberty is preceded byadrenarche, marking an increase of adrenal androgen production between ages 6–10. Adrenarche is sometimes accompanied by the early appearance of axillary and pubic hair. The first androgenic hair resulting from adrenarche can be also transient and disappear before the onset of true puberty.
The onset of puberty is associated with highGnRH pulsing, which precedes the rise in sex hormones,LH andFSH.[17] Exogenous GnRH pulses cause the onset of puberty.[18] Brain tumors which increase GnRH output may also lead topremature puberty.[19]
The cause of the GnRH rise is unknown.Leptin might be the cause of the GnRH rise. Leptin has receptors in thehypothalamus which synthesizes GnRH.[20] Individuals who are deficient in leptin fail to initiate puberty.[21] The levels of leptin increase with the onset of puberty, and then decline to adult levels when puberty is completed. The rise in GnRH might also be caused by genetics. A study discovered that a mutation in genes encoding bothneurokinin B as well as the neurokinin B receptor can alter the timing of puberty.[22] The researchers hypothesized that neurokinin B might play a role in regulating the secretion ofkisspeptin, a compound responsible for triggering direct release of GnRH as well as indirect release of LH and FSH.[22]
Effects of early and late puberty onset
Several studies about puberty have examined the effects of an early or a late onset of puberty in males and females. In general, females who enter puberty late experience positive outcomes in adolescence and adulthood, while females who enter puberty early experience negative outcomes. Males who have earlier pubertal timing generally have more positive outcomes in adulthood but more negative outcomes in adolescence, while the reverse is true for later pubertal timing.[23]
Females
Outcomes have generally indicated that early onset of puberty in females can be psychologically damaging. The main reason for this detrimental effect is the issue ofbody image. As they physically develop, gaining weight in several areas of the body, early-maturing females usually look larger than females who have not yet entered puberty. A result of the social pressure to be thin, the early-maturing females develop a negative view of their body image. In addition, people may tease the females about their visible breasts, forcing the early-maturing female to hide her breasts by dressing differently. Embarrassment about a more developed body may also result in the refusal to undress for gym. These experiences lead to lower self-esteem, more depression and poorer body image in these early-maturing females.[23]
Furthermore, as physical and emotional differences set them apart from people in their same age group, early-maturing females develop relationships with older people. For instance, some early-maturing females have older malefriends, "attracted to the females' womanly physique and femaleish innocence."[23] While having an older malefriend might improve popularity among peers, it also increases the risk of alcohol and drug use, increased sexual relations (often unprotected), eating disorders and bullying.[23]
Generally, later onset of puberty in females produces positive outcomes. They exhibit positive behaviors in adolescence that continue to adulthood.[23]
Males
In the past, early onset of puberty in males has been associated with positive outcomes, such as leadership in high school and success in adulthood.[24] However, recent studies have revealed that the risks and problems of early maturation in males might outweigh the benefits.[23]
Early-maturing males develop "more aggressive, law-breaking, and alcohol abusing" behaviors, which result in anger towards parents and trouble in school and with the police. Early puberty also correlates with increased sexual activity and a higher instance of teenage pregnancy, both of which can lead to depression and other psychosocial issues.[23]
On the other hand, late-maturing males develop lower self-esteem and confidence and generally have lower popularity among peers, due to their less-developed physiques. Also, they experience problems with anxiety and depression and are more likely to be afraid of sex than other males.[23]
In males, puberty begins with the enlargement of the testicles andscrotum. The penis also increases in size, and a male develops pubic hair. A male's testicles also begin makingsperm. The release ofsemen, which contains sperm and other fluids, is calledejaculation.[25] During puberty, a male's erect penis becomes capable of ejaculating semen and impregnating a female.[26][27] A male's first ejaculation is an important milestone in his development.[28] On average, a male's first ejaculation occurs at age 13.[29] Ejaculation sometimes occurs during sleep; this phenomenon is known as anocturnal emission.[25]
Testicular size
Five Tanner stages of male genitalia;The Adolescent Period
In males, testicular enlargement is the first physical manifestation of puberty (and is termedgonadarche).[30]Testes in prepubertal males change little in size from about 1 year of age to the onset of puberty, averaging about 2–3 cm in length and about 1.5–2 cm in width. The size of the testicles is among the parameters of theTanner scale for male genitals, from stage I which represents a volume of less than 1.5 ml, to stage V which represents a testicular volume of greater than or equal to 20 ml. Testicular size reaches maximal adult size about 6 years after the onset of puberty. While 18–20 cm3 is an average adult size, there is wide variation in testicular size in the normal population.[31] After the male's testicles have enlarged and developed for about one year, the length and then the breadth of the shaft of thepenis will increase and theglans penis andcorpora cavernosa will also start to enlarge to adult proportions.[32]
Development of a male from childhood to the end of puberty
Erections
Erections during sleep or when waking up are medically known asnocturnal penile tumescence and colloquially referred to asmorning wood.[33] The penis can regularly get erect during sleep and men or males often wake up with an erection.[34] Once a male reaches histeenage years, erections occur much more frequently due to puberty.[35] Erections can occur spontaneously at any time of day, and if clothed may cause a bulge or "hump". This can be disguised or hidden by wearing close-fitting underwear, a long shirt and baggier clothes.[36] Erections are common for male prepubescent children and infants, and can even occur before birth.[37] Spontaneous erections, also known as involuntary or unwanted erections, are normal. Such erections can be embarrassing if they happen in public, such as a classroom or living room.[38][39]
Foreskin retraction
During puberty, if not before, the tip and opening of a male'sforeskin becomes wider, progressively allowing for retraction down the shaft of thepenis and behind theglans, which ultimately should be possible without pain or difficulty. The membrane that bonds the inner surface of the foreskin with the glans disintegrates and releases the foreskin to separate from the glans. The foreskin then gradually becomes retractable.[40]
Research by Øster (1968) found that with the onset and continuation of puberty, the proportion of males able to pull back their foreskins increased. At ages 12–13, Øster found that only 60% of males were able to retract their foreskins; this increased to 85% by ages 14–15, and 95% by 16–17. He also found that 1% of those unable to fully retract experiencedphimosis at ages 14–17, the remainder were partially able to.[40] The findings were supported by further research by Kayabaet al (1996) on a sample of over 600 males,[41] and Ishikawa and Kawakita (2004) found that by age 15, 77% of their sample of males could retract their foreskins.[42] Beaugé (1997) reports that males may assist the development of retractile foreskin by manual stretching.[43]
Once a male is able to retract his foreskin, penile hygiene should become an important feature of his routine body care. Although theAmerican Academy of Pediatrics states there is "little evidence to affirm the association between circumcision status and optimal penile hygiene",[44] various studies suggest that males be educated about the role of hygiene, including retracting the foreskin while urinating and rinsing under it and around the glans at each bathing opportunity. Regular washing under the foreskin was found by Krueger and Osborn (1986) to reduce the risk of numerous penile disorders,[45] however Birley et al. (1993) reports excessive washing with soap should be avoided because it dries the oils out of the tissues and can cause non-specificdermatitis.[46]
Male at 11.3 yo (prepubertal), 12.5 yo, 14.9 yo and 16.3 yo (post pubertal)
In the months and years following the appearance of pubic hair, other areas of skin that respond to androgens may developandrogenic hair. The usual sequence is:underarm (axillary) hair,perianal hair,upper lip hair,sideburn (preauricular) hair, periareolar hair, and thebeard area.[47] As with most human biological processes, this specific order may vary among some individuals. 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 racial groups. Facial hair is often present in late adolescence, but may not appear until significantly later.[48][49] Facial hair will continue to get coarser, darker and thicker for another 2–4 years after puberty.[48] Some men do not develop full facial hair for up to 10 years after the completion of puberty.[48]
Under the influence of androgens, thelarynx (or voice box)grows in both sexes. This growth is far more prominent in males, causing the male voice to drop and deepen, sometimes abruptly but rarely "overnight", about oneoctave, because the longer and thickervocal folds have a lowerfundamental frequency. Before puberty, the larynx of males and females is about equally small.[50]
The first physical sign of puberty in females is usually a firm, tender lump under the center of theareola of one or bothbreasts, occurring on average at about 10½ years of age.[51] This is referred to asthelarche. By the widely usedTanner staging of puberty, this is stage 2 of breast development (stage 1 is a flat, prepubertal breast). Within 6–12 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 andnipples 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.[52]
Pubic hair
Pubic hair is often the second noticeable change in puberty, usually within a few months of thelarche.[53] It is referred to aspubarche. The pubic hairs are usually visible first along thelabia. The first few hairs are described as Tanner stage 2.[52] Stage 3 is usually reached within another 6–12 months, when the hairs are too numerous to count and appear on thepubic mound as well. By stage 4, the pubic hairs densely fill the "pubic triangle". Stage 5 refers to spread of pubic hair to thethighs and sometimes asabdominal hair upward towards thenavel. In about 15% of females, the earliest pubic hair appears before breast development begins.[53]
Vagina, uterus, ovaries
Perineal skinkeratinizes due to effect of estrogen increasing its resistance to infection. Themucosalsurface of thevagina also changes in response to increasing levels ofestrogen, becoming thicker and duller pink in color (in contrast to the brighter red of the prepubertal vaginal mucosa).[54] Mucosa changes into a multilayered structure with superficial layer of squamous cells. Estrogen increase glycogen content invaginal epithelium, which in future plays important part in maintainingvaginal pH. Whitish secretions (physiologicleukorrhea) are a normal effect of estrogen as well.[51] In the two years following thelarche, theuterus,ovaries, and thefollicles in the ovaries increase in size.[55] The ovaries usually contain small follicularcysts visible byultrasound.[56][57]
Menstruation and fertility
The firstmenstrual bleeding is referred to asmenarche, and typically occurs about two years afterthelarche.[53] The average age of menarche is 12½ in theUnited States.[58] Most American females experience their first period at 11, 12 or 13, but some experience it earlier than their 11th birthday and others after their 14th birthday. In fact, anytime between 8 and 16 is normal. InCanada, the average age of menarche is 12.72,[59] and in theUnited Kingdom it is 12.9.[60] The time between menstrual periods (menses) is not always regular in the first two years after menarche.[61]Ovulation is necessary forfertility, but may or may not accompany the earliest menses.[62] In postmenarchal females, about 80% of the cycles were anovulatory in the first year after menarche, 50% in the third year and 10% in the sixth year.[61] Initiation of ovulation after menarche is not inevitable. A high proportion of females with continued irregularity in the menstrual cycle several years from menarche will continue to have prolonged irregularity and anovulation, and are at higher risk for reduced fertility.[63]
Body shape, fat distribution, and body composition
Development of a female from childhood to the end of puberty
During this period, also in response to rising levels of estrogen, the lower half of thepelvis and thuships widen (providing a largerbirth canal).[52][64] 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. On average, at 10 years, females have 6% more body fat than males.[65]
Body odor and acne
Rising levels ofandrogens can change thefatty acid composition ofperspiration, resulting in a more "adult"body odor. This often precedes thelarche and pubarche by one or more years. Another androgen effect is increased secretion of oil (sebum) from the skin. This change increases the susceptibility toacne, a skin condition that is characteristic of puberty. Acne varies greatly in its severity.[66]
Changes of the vulva initiated by estradiol as well as its direct effects also appear to influence the functioning of the lower urinary tract.[68][69]
Underarm hair
Hair growth develops under the arms, starting out sparse before thickening and darkening over time.[70]
Variations
Variations of the initial and final height of three males from 12 years old to the end of their growth spurt
In a general sense, the conclusion of puberty is reproductive maturity. Criteria for defining the conclusion may differ for different purposes: attainment of the ability to reproduce, achievement of maximal adult height, maximal gonadal size, or adult sex hormone levels. Maximal adult height is achieved at an average age of 15 years for an average female and 18 years for an average male. Potential fertility (sometimes termednubility) usually precedes completion of growth by 1–2 years in females and 3–4 years in males. Stage 5 typically represents maximal gonadal growth and adult hormone levels.[citation needed]
Age of onset
The definition of the onset of puberty may depend on perspective (e.g., hormonal versus physical) and purpose (establishing population normal standards, clinical care of early or late pubescent individuals, etc.). A common definition for the onset of puberty is physical changes to a person's body.[13] These physical changes are the first visible signs of neural, hormonal, and gonadal function changes.
The age at which puberty begins varies between individuals; usually, puberty begins between 10 and 13 years of age. The age at which puberty begins is affected by both genetic factors and by environmental factors such as nutritional state and social circumstances.[71] An example of social circumstances is theVandenbergh effect; a juvenile female mouse who has significant interaction with adult male mice will enter puberty earlier than juvenile females who are not socially overexposed to adult males.[72]
The average age at which puberty begins may be affected byethnicity as well. For example, the average age ofmenarche in various populations surveyed has ranged from 12[58][59][60] to 18 years. The earliest average onset of puberty is for African-American females and the latest average onset for high altitude subsistence populations in Asia. However, 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 females in the population, and the width of the spread may reflect unevenness of wealth and food distribution in a population.
Researchers have identified an earlier age of the onset of puberty. However, they have based their conclusions on a comparison of data from 1999 with data from 1969. In the earlier example, the sample population was based on a small sample of white females (200, from Britain). The later study identified as puberty as occurring in 48% of African-American females by age nine, and 12% of white females by that age.[73]
One possible cause of a delay in the onset of puberty past the age 14 in females and 15 in males isKallmann syndrome, a form ofhypogonadotropic hypogonadism (HH). Kallmann syndrome is also associated with a lack of sense of smell (anosmia). Kallmann syndrome and other forms of HH affect both men and women. It is caused by a failure inHPG axis at puberty which results in low or zerogonadotropin (LH andFSH) levels with the subsequent result of a failure to commence or complete puberty, secondaryhypogonadism andinfertility.[74][75]
Comparison of two individual with vast difference in the age of onset of puberty:
Two males from 11.5 to 16.6 years oldTwo females from 8.0 to 14.5 years old
Historical shift
The average age at which the onset of puberty occurs has dropped significantly since the 1840s.[76][77][78]
A 2006 study in Denmark found that puberty, as evidenced by breast development, started at an average age of 9 years and 10 months, a year earlier than when a similar study was done in 1991. Scientists believe the phenomenon could be linked to obesity or exposure to chemicals in the food chain, and is putting females at greater long-term risk of breast cancer.[79]
Genetic influence and environmental factors
Various studies have found direct genetic effects to account for at least 46% of the variation of timing of puberty in well-nourished populations.[80][81][82][83] The genetic association of timing is strongest between mothers and daughters. The specificgenes affecting timing are not yet known.[80] Among the candidates is anandrogen receptor gene.[84]
Researchers have hypothesized that early puberty onset may be caused by certain hair care products containing estrogen or placenta, and by certain chemicals, namelyphthalates, which are used in many cosmetics, toys, and plastic food containers.[73]
Hormones and steroids
There is theoretical concern, and animal evidence, that environmental hormones andchemicals may affect aspects of prenatal or postnatal sexual development in humans.[85]
Bisphenol A (BPA) is a chemical used to make plastics, and is frequently used to make baby bottles, water bottles, sports equipment, medical devices, and as a coating in food and beverage cans. Scientists are concerned about BPA's behavioral effects on fetuses, infants, and children at current exposure levels because it can affect the prostate gland, mammary gland, and lead to early puberty in females. BPA mimics and interferes with the action of estrogen—an important reproduction and development regulator. Itleaches out of plastic into liquids and foods, and theCenters for Disease Control and Prevention (CDC) found measurable amounts of BPA in the bodies of more than 90 percent of the U.S. population studied. The highest estimated daily intakes of BPA occur in infants and children. Many plastic baby bottles contain BPA, and BPA is more likely to leach out of plastic when its temperature is increased, as when one warms a baby bottle or warms up food in the microwave.[86]
Nutritional influence
Nutritional factors are the strongest and most obvious environmental factors affecting timing of puberty.[80]
Obesity influence and exercise
Scientific researchers have linked earlyobesity with an earlier onset of puberty in females. They have cited obesity as a cause of breast development before nine years and menarche before twelve years.[87] Early puberty in females can be a harbinger of later health problems.[88]
The endocrinereproductive system consists of thehypothalamus, thepituitary, thegonads, and theadrenal glands, with input and regulation from many other body systems. True puberty is often termed "central puberty" because it begins as a process of thecentral nervous system. A simple description of hormonal puberty is as follows:
Rising levels of estradiol and testosterone produce the body changes of female and male puberty.
The onset of this neurohormonal process may precede the first visible body changes by 1–2 years.
Components of the endocrine reproductive system
Thearcuate nucleus of thehypothalamus is the driver of the reproductive system. It hasneurons which generate and release pulses of GnRH into the portal venous system of thepituitary gland. The arcuate nucleus is affected and controlled by neuronal input from other areas of the brain and hormonal input from thegonads,adipose tissue and a variety of other systems.
Thepituitary gland responds to the pulsed GnRH signals by releasing LH and FSH into the blood of the general circulation, also in a pulsatile pattern.
LH (luteinizing hormone) is a largerprotein hormone secreted into the general circulation by gonadotrope cells of the anteriorpituitary gland. The main target cells of LH are theLeydig cells oftestes and thetheca cells of theovaries. LH secretion changes more dramatically with the initiation of puberty than FSH, as LH levels increase about 25-fold with the onset of puberty, compared with the 2.5-fold increase of FSH.
Leptin is a protein hormone produced byadipose tissue. Its primary target organ is thehypothalamus. The leptin level seems to provide thebrain a rough indicator of adipose mass for purposes of regulation ofappetite andenergy metabolism. It also plays a permissive role in female puberty, which usually will not proceed until an adequate body mass has been achieved.
Endocrine perspective
Theendocrine reproductive system becomes functional by the end of the firsttrimester of fetal life. The testes and ovaries become briefly inactive around the time of birth but resume hormonal activity until several months after birth, when incompletely understood mechanisms in the brain begin to suppress the activity of the arcuate nucleus. This has been referred to as maturation of the prepubertal "gonadostat", which becomes sensitive to negative feedback bysex steroids. The period of hormonal activity until several months after birth, followed by suppression of activity, may correspond to the period of infant sexuality, followed by alatency stage, whichSigmund Freud described.[91]
Neurons of the arcuate nucleus secretegonadotropin releasing hormone (GnRH) into the blood of the pituitary portal system. An American physiologist, Ernst Knobil, found that the GnRH signals from the hypothalamus induce pulsed secretion of LH (and to a lesser degree, FSH) at roughly 1–2 hour intervals. The LH pulses are the consequence of pulsatile GnRH secretion by the arcuate nucleus that, in turn, is the result of anoscillator or signal generator in the central nervous system ("GnRH pulse generator").[92] In the years preceding physical puberty,Robert M. Boyar discovered that the gonadotropin pulses occur only during sleep, but as puberty progresses they can be detected during the day.[93] By the end of puberty, there is little day-night difference in the amplitude and frequency of gonadotropin pulses.
Some investigators have attributed the onset of puberty to aresonance of oscillators in the brain.[94][95][96] By this mechanism, the gonadotropin pulses that occur primarily at night just before puberty representbeats.[97][98][99][100]
An array of "autoamplification processes" increases the production of all of the pubertal hormones of the hypothalamus, pituitary, and gonads.[101]
Notes: Values are mean plasma levels, with ranges in parentheses.a = Pre-adrenarche.b = Post-adrenarche. (Adrenarche, or increasedadrenal androgen section, occurs as a separate event and can precede puberty onset by 1 to 2 years.)Sources:[102]
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Sizonenko, PC.Role of sex steroids during development—integrationArchived 2013-07-04 at theWayback Machine. in Bourguignon, Jean Pierre & Tony M. Plant. The Onset of Puberty in Perspective: Proceedings of the 5th International Conference on the Control of the Onset of Puberty, Held in Liège, Belgium, 26–28 September 1999. Elsevier. Amsterdam & New York 2000.ISBN0-444-50296-3. pp 299–306.
Tanner JM, Davies PS (1985). "Clinical longitudinal standards for height and height velocity for North American children".The Journal of Pediatrics.107 (3):317–29.doi:10.1016/S0022-3476(85)80501-1.PMID3875704. Highly useful growth charts with integrated standards for stages of puberty.
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