What causes puberty to begin?

What causes puberty to begin?

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I understand that puberty begins when hormones are released from the pituitary gland as instructed by the hypothalamus, but why does the hypothalamus instruct this to occur?

How is this process timed by the body?

Sex abuse triggers early puberty and its problems

Puberty can be a tough time for all youth, but for girls who have been sexually abused, it spells double trouble. Sexually abused girls reach puberty before other girls, a new study finds, and early puberty increases their risk of having emotional problems.

“Early maturing girls are already more vulnerable to mood problems than other kids, but this risk seems to be magnified for girls with histories of sexual abuse,” said Jane Mendle, assistant professor of human development in Cornell’s College of Human Ecology, whose study was published online (Aug. 28) in the Journal of Research on Adolescence ahead of print.

“Girls who reach puberty ahead of peers are substantially more likely to be targets of peer sexual harassment and receive a high number of unsolicited comments on their bodies,” said first author Mendle.

For those with histories of sexual abuse (about one in five girls in the United States), these challenges and pressures may become a tipping point for emotional difficulties such as depression and anxiety, she said.

Mendle and colleagues studied 100 girls in foster care, all of whom had experienced maltreatment early in childhood. They looked at the type of maltreatment (physical abuse, sexual abuse or neglect), emotional symptoms and level of physical maturity reported at two points, two years apart.

The team found no direct effects of abuse on the girls’ emotional symptoms. Rather, they found that the number of sexual abuse instances, but not physical abuse or neglect instances, was linked to earlier pubertal timing. And it was these earlier developing girls that had more symptoms of depression, anxiety and social withdrawal. The study showed that girls’ emotional problems were directly related to their experiences at puberty – not to what happened to them early in life, the authors concluded.

“In addition to individual interventions [to help early maturing girls], another target might be our collective social response to early puberty,” said Mendle.

“Peers, caregivers, teachers and other adults have a tendency to react to children based on their observable – rather than chronological – age. Those reactions can be very powerful for how girls respond and interpret the challenges of growing up.”

The study “Linking Childhood Maltreatment With Girls’ Internalizing Symptoms: Early Puberty as a Tipping Point,” was co-authored by Leslie D. Leve and Mark Van Ryzin of the University of Oregon and Misaki N Natsuaki of the University of California, Riverside. It was supported in part by the National Institute on Drug Abuse.

Karene Booker is an extension support specialist in the Department of Human Development.

The only obvious difference between boys and girls at birth is their reproductive organs. However, even the reproductive organs start out the same in both sexes.

Development Before Birth

In the first several weeks after fertilization, males and females are essentially the same except for their chromosomes. Females have two X chromosomes (XX), and males have an X and aY chromosome (XY). Then, during the second month after fertilization, genes on the Y chromosome of males cause the secretion of testosterone. Testosterone stimulates the reproductive organs to develop into male organs. (Without testosterone, the reproductive organs always develop into female organs.) Although boys have male reproductive organs at birth, the organs are immature and not yet able to produce sperm or secrete testosterone.

Puberty and Its Changes

The reproductive organs grow very slowly during childhood and do not mature until puberty. Puberty is the period during which humans become sexually mature. In the U.S., boys generally begin puberty at about age 12 and complete it at about age 18.

What causes puberty to begin? The hypothalamus in the brain &ldquotells&rdquo the pituitary gland to secrete hormones that target the testes. The main pituitary hormone involved is luteinizing hormone (LH). It stimulates the testes to secrete testosterone. Testosterone, in turn, promotes protein synthesis and growth. It brings about most of the physical changes of puberty, some of which are shown in Figure below. In addition to the changes shown below, during puberty male facial hair begins to grow, the shoulders broaden, and the male voice deepens.

Some of the changes that occur in boys during puberty are shown in this figure. Pubic hair grows, and the penis and testes both become larger.

Adolescent Growth Spurt

Another obvious change that occurs during puberty is rapid growth. This is called the adolescent growth spurt. In boys, it is controlled by testosterone. The rate of growth usually starts to increase relatively early in puberty. At its peak rate, growth in height is about 10 centimeters (almost 4 inches) per year in the average male. Growth generally remains rapid for several years. Growth and development of muscles occur toward the end of the growth spurt in height. Muscles may continue to develop and gain strength after growth in height is finished.

Girls' Early Puberty: What Causes It, And How To Avoid It

Today in the U.S., about 16 percent of girls enter puberty by the age of 7, and about 30 percent by the age of 8. A recent study determined that the number of girls entering puberty (defined by breast development) at these early ages has increased markedly between 1997 and 2010.[1]

Trends in Age at Menarche
The average age at menarche in Western countries began declining during the early part of the 20th century due to increased consumption of animal products and increasing calorie intake the decline slowed in the 1960s, and now in the U.S. there has been a more recent surge in early puberty starting in the mid-1990s.[2]

In Europe, in 1830, the average age at menarche was 17. Similarly in the 1980s in rural China, the average age at menarche was 17.3 In the U.S. in 1900, the average was 14.2. By the 1920s, average age at menarche in the U.S. had fallen to 13.3 and by 2002, it had reached 12.34.[4] Similar trends are occurring in other Western nations.[5,6] For example, age at menarche in Ireland has declined from 13.52 in 1986 to 12.53 in 2006.[7] In Italy, a recent study showed that girls' age at menarche was on average 3 months earlier than their mothers'.[8]

Taking all this data together, we can estimate that the normal, healthy age at menarche under conditions of excellent nutrition without caloric excess would probably fall somewhere between 15 and 18. But today in the U.S., about half of girls begin developing breasts before age 10, and the average age at menarche is less than 12 ½ and still declining.

Why Is this happening?
The neurological and hormonal systems that regulate pubertal timing are complex, but research has identified a number of environmental factors that may be contributing to the decline in age at puberty:

Increasing rates of childhood overweight and obesity
Several studies have found associations between higher childhood BMI and earlier puberty in girls.[4,9-11] Excess body fat alters the levels of the hormones insulin, leptin, and estrogen, and these factors are believed to be responsible for the acceleration of pubertal timing by obesity. Also, physical inactivity may decrease melatonin levels, which can also affect signals in the brain that trigger pubertal development.[4, 12]

Increased animal protein intake
Higher total protein, animal protein, and meat intake in children age 3-7 have been associated with earlier menarche in multiple studies.[13-15] In contrast, higher vegetable protein intake at age 5-6 is associated with later menarche.[15] High protein intake elevates IGF-1 levels and promotes growth, which could accelerate the onset of puberty - IGF-1 contributes to pubertal development on its own and in part by its involvement in estradiol signaling.[4,16] Meat and dairy consumption in children may also reflect ingestion of environmental endocrine-disrupting chemicals (EDCs) that have accumulated in animal tissues (see EDCs below).

Other dietary factors:
High dairy consumption is associated with earlier than average menarche.[17] Soft drink consumption is associated with early menarche.[18]

Children with lower nutrient diets (based on analysis of macronutrients, vitamins, minerals, and certain whole foods) tend to enter puberty earlier.[19] Overall our modern diet rich in processed foods, dairy, processed meats and fast food is disruptive to normal development and aging. Early puberty is an early sign of premature aging.

Exposure to endocrine-disrupting chemicals (EDCs)
EDCs are hormonally active synthetic chemicals that either mimic, inhibit, or alter the action of natural hormones. These chemicals are ubiquitous in our environment, and are considered by scientists to be a significant public health concern. Although EDCs are thought to pose a threat to adults as well, children's bodies are more sensitive to exposure to exogenous hormones.[20]

Chemicals are not currently tested for their endocrine disruption potential before they are approved for use and enter our environment, and there are endocrine disruptors in a vast array of products we come into contact with every day, including organochlorine pesticides, plastics, fuels, and other industrial chemicals.[21]

The substances of most concern currently are BPA and phthalates. BPA is one of the highest volume chemicals produced in the world. It is used in the manufacture of polycarbonate plastics, such as rigid cups, water bottles and food storage containers BPA is also found in the linings of food cans and dental sealants. BPA can leach from containers into food and beverages, especially during heating and washing.[4] BPA exposure is associated with early puberty in girls.[22]

Phthalates are chemicals used to make PVC plastics more flexible, and are found in a variety of products including toys, food packaging, hoses, raincoats, shower curtains, vinyl flooring, wall coverings, lubricants, adhesives, detergents, nail polish, hair spray, and shampoo. Phthalates are associated with early breast development in girls.[22, 23] They are considered chemicals of concern to the EPA and may be phased out -- some phthalates have already been removed from children's toys and cosmetics. [24]

Additional EDCs that have been associated with dysregulation of pubertal timing include industrial chemicals such as PCBs, pesticides such as DDT and endosulfan, the flame retardant PBB, and dioxins and furans, which are formed during incineration of waste, chlorine bleaching of paper, and chemical manufacturing. [22,23,25,26]

It is important to note that EDCs break down very slowly and accumulate in the fatty tissues of animals, so animal foods contain higher levels of these chemicals than plant foods.

Why is this troublesome?
The most significant and alarming consequence of early maturation is an increased risk for breast cancer in adulthood. Early menarche is an established risk factor for breast cancer, and this is believed to be due to the extended lifetime exposure to ovarian hormones.[10,27,28] Similarly, exposure to EDCs during childhood is associated with hormonal cancers, such as breast and testicular cancers.[29-31]

Girls of 7, 8 or 9 years old are not emotionally or psychologically equipped to handle puberty. As such, earlier puberty is also associated with a higher risk of psychological problems during adolescence such as anxiety, depression, and eating disorders. Girls who mature earlier are also more likely to take part in risky behaviors like smoking and alcohol use.[4,12]

  • Children's diets should focus on whole plant foods rather than animal foods -- this will keep protein intake in a safe range and reduce their consumption of EDCs.
  • Minimize dairy products in children's diets -- use almond and hemp milks instead of cows' milk
  • Encourage children to exercise and exercise with them.
  • Minimize processed foods -- these are calorie-dense and nutrient-poor, and they promote obesity and other diseases.
  • Children's diets should include a wide variety of natural plant foods as possible including, green vegetables, squashes, corn, carrots, tomatoes, onions, mushrooms, nuts, seeds, avocados, beans, fruits and whole grains. This means that healthy eating is a lifetime event.
  • Buy organic produce when possible to avoid synthetic pesticides.
  • Minimize children's exposure to BPA: Avoid using of rigid polycarbonate plastics (recycling label #7) whenever possible Do not use plastic water bottles if they are old or scratched Do not microwave in plastic containers Minimize the use of canned foods and avoid canned infant formulas.32
  • Minimize children's exposure to phthalates:Avoid plastics marked with recycling label #3 (PVC) whenever possible Check ingredient lists on personal care products for phthalates. Also be aware that "fragrance" listed as an ingredient often means that the products contains phthalates. For more information, visit the Environmental Working Group's guide to children's personal care products.

To conclude, the earlier occurrence of puberty is an ominous event that we can stop. We can even win the war on breast cancer in America and prevent millions of young females from developing it. The answer however, must begin in the way we feed ourselves and our children. The most effective type of health care is vigilant and excellent self care.

    National Organization for Rare Disorders. (2016). Precocious Puberty. Retrieved May 15, 2018, from Wei, C., Davis, N., & Honour, J. (2016). The investigation of children and adolescents with abnormalities of pubertal timing. Annals of Clinical Biochemistry: International Journal of Laboratory Medicine. 54, 20-32. Retrieved May 16, 2018, from

Puberty is the body's natural process of sexual maturation. Puberty's trigger lies in a small part of the brain called the hypothalamus, a gland that secretes gonadotropin-releasing hormone (GnRH). GnRH stimulates the pituitary gland, a pea-sized organ connected to the bottom of the hypothalamus, to emit two hormones: luteinizing (pronounced LOO-tee-uh-nize-ing) hormone (LH) and follicle-stimulating hormone (FSH). These two hormones signal the female and male sex organs (ovaries and testes, respectively) to begin releasing the appropriate sex hormones, including estrogens and testosterone, which launch the other signs of puberty in the body. 1

Delayed Puberty

If your son hasn't started puberty by the age of 14, which means that his testicles and penis haven't started to grow yet, this is considered delayed puberty. The most common cause is called constitutional delayed puberty.   Most boys who are constitutionally delayed are totally healthy and will go through puberty eventually.

More than two-thirds of boys inherit this from one or both of their parents who also started puberty late.   In boys, this can be defined as having no increase in testicle size by the age of 14 years old or continuing undergo puberty for more than five years after the start.

In girls, delayed puberty is starting menstruation after the age of 16 years. The majority of boys who are constitutionally delayed are also short compared to other boys their age, but this is just because they haven't had their growth spurt yet.  

If your son has a chronic illness like sickle cell disease, inflammatory bowel disease, or cystic fibrosis, puberty may also begin later than normal.

A small number of boys have a condition called isolated gonadotropin deficiency (IGD), which means that they don't produce adequate amounts of the hormones LH and FSH. This condition typically begins at birth and is typically treated with testosterone injections.

An even smaller number of boys have something going on with their testicles that's causing puberty to be delayed. Testosterone is the main treatment for issues of this sort.

Adolescence and the Problems of Puberty

Adolescence and puberty are not the same.

Adolescence is that 10- to 12-year period of social and psychological growth that transforms the dependent child (beginning in late elementary or early middle school) into a functionally independent young adult in his or her early to mid-twenties.

Puberty is the one to three-year process of hormonal and physical change that causes the young person to reach sexual maturity, girls usually entering it about a year earlier than boys.

Among other changes wrought by puberty, there are growth spurts that create bigger bodies to manage. For girls, hips broaden, breasts swell, menstruation begins, and they can produce eggs. For boys, muscles enlarge, voice drops, ejaculation begins, and they can produce sperm. For both males and females, there is more hair around sex organs, more body odor, and more active skin glands that can create acne.

Now young people as young as 10 to 14 are capable of participating in sexual reproduction, which doesn't mean that they immediately want to fulfill that potentiality. What it does mean, however, is that parents do need to start educating their son or daughter about socially managing sexual maturity and delaying sexual activity in a popular culture that glamorizes looking and acting sexual in every way.

This is no time for a young person to be uninformed about what is going on in their bodies because in ignorance they will believe they are unique and wonder what is wrong with them, when nothing is. This is a time for parents to explain the process of puberty that unfolds for everyone and what changes to expect.

An easy way to do this is for parents to search online for sites explaining puberty, find one that they like, and then read the information with their son or daughter, inviting any questions the young person may have. Normalize the process so the young person doesn't "abnormalize" themselves.

Adolescence does not depend on puberty to start. In fact, in most cases, adolescence begins first. Parents notice the negative attitude (more criticism and complaining), the passive and active resistance (more delay and arguments), and the testing of limits (more seeing what can be gotten away with) that are the hallmarks of early adolescent change. But when puberty does begin, the adolescent transformation becomes emotionally intensified and more complex.

Puberty now creates two problems in one. First, it creates a process problem: how to manage the physical changes that are besetting their bodies. This is the problem of self-consciousness. And second, it creates an outcome problem: how to act young manly or young womanly. This is the problem of sex-role definition.

Start with the problem of self-consciousness. For most young people, puberty catches them at a bad time — during the early adolescent years (around ages 9-13) when they are separating from the shelter of childhood and begin striving for social belonging and place among their society of peers. Already feeling adrift from family and at sea in this brave new world of more social independence, puberty demonstrates how they are also out of control of their bodies.

Developmental insecurity and early adolescence go hand in hand. For most young people, puberty is the enemy of self-esteem. It changes how they look at a time when physical appearance becomes more important for social acceptance and social standing.

As body shape and characteristics alter, they feel more vulnerable on that account, whether they are physically maturing too fast or not fast enough. This is the period when self-examination is microscopic, when any new blemish can be a source of misery, when it takes much longer to "get ready" to go out, when what to wear and how to groom absorb protracted attention.

At home, parents must remember that the changes of puberty are no laughing matter. The rule for parents is there must be no teasing, no joking, no making fun of self-preoccupation, physical appearance, bodily change, or choice of dress. There is enough of this torment from peers who are all suffering from similar insecurities themselves.

Early adolescence is an age of intolerance, where perceived differences or departures from the dominant or desired norm are not treated kindly. Now a young woman or young man can be teased and picked on for not looking womanly or manly enough. A painfully self-conscious early adolescent can take this social cruelty very personally. "What's wrong with me?" "I hate how I look!" "I'll never fit in!" Self-esteem can plummet when being teased causes a young person to become self-rejecting.

Or there can be a vulnerability to rumoring that can come from appearing so mature so young — peers gossiping that because you look so sexually mature you are prepared to act that way. So now you have a sexual social reputation.

At this juncture, parents need to help the young person evaluate this cruelty for what it is. "Being teased or rumored this way shows nothing wrong with you, but it shows a lot wrong about them. They are ridiculing what they fear being attacked about themselves, and they are choosing to at mean. This mistreatment is about them, not about you."

Now consider the problem of sex-role definition. While adolescence begins growth toward more independence, puberty adds another dimension to this journey — the need to claim one's young manhood or young womanhood. But where are young people supposed to learn these definitions?

Certainly, there are models in the family if older siblings and parents are available to provide salient examples to follow. Even so, these are not the most commanding images at hand. It is the cultural ideals for being a man and being a woman that young people find most alluring, ideals portrayed in the images and messages and icons that media advertising and entertainment constantly communicate.

To approximate these young manly and young womanly attributes means incorporating some of them into one's desired appearance. So come puberty, the social/sexual stereotypes kick in as young women worry about weight and thinning down their bodies by dieting, and young men worry about muscle size and strengthening their bodies by lifting weights.

And now social role definition is added to the mix. According to stereotype, the male is encouraged to be sexual aggressor, the female is encouraged to be sexual attractor. You can literally see these images played out at the middle and high school football games, for example, where young men bulk up to play a collision sport in front of young women who dress and dance in form-fitting clothing to cheer them on. These are very incomplete sex role definitions.

After puberty, young women who are not deemed attractive enough by their peers, and young men who are not deemed aggressive enough by their peers, can feel punished by being told and shown how they are not measuring up — girls for being too fat, boys for being too weak.

Hopefully, at this juncture, parents can help their son or daughter escape the pressure of these dehumanizing sex role definitions by explaining a more healthy way to grow. For example, they could say something like this:

"Don't pay too much attention to what the popular sexual stereotypes have to say about how you should be because when it comes to appreciating human variation they're very restrictive. The truth is, there are as many good ways to be a woman as there are women. There are as many good ways to be a man as there are men. And your job is to discover and develop a good way to be womanly or manly that fits and fulfills the authentic person you want to become."

A final word needs to be said about early puberty, a reality that affects girls a significant number of girls. When puberty begins prior to the usual onset of adolescence (around ages 9-13) it can put a girl at a serious disadvantage, for several reasons:

The Truth Behind Early Puberty

Kids grow up fast these days, but many parents are worried it's too fast when little Sally needs a training bra at age six and Timmy sports a moustache at ten.

The phenomenon is called precocious puberty. Some doctors think it is happening with greater frequency since the 1990s, especially among girls. Many factors have been implicated: hormones in food, hormone-mimicking pollutants in the waterways, and even social issues, such as sex in the mass media and single motherhood.

Yet not all scientists are convinced of an actual rise in precocious puberty reports have been anecdotal, and studies have been small or inconclusive.

And while all of these possible causes to this possible problem are plausible, doctors say the likely cause, if any, is childhood obesity: That is, it's not what's in the food but food itself, and lots of it, in all it's fatty-sugary glory, coupled with inactivity.

10-year-old women

Precocious puberty is as old as puberty itself. Some kids&mdashfor reasons usually genetic but sometimes as a result of a brain tumor or pituitary gland disorder&mdashmature sexually faster than others. I distinctly remember a boy in my Catholic grade school who seemed to have grown a moustache overnight, at age 10. He was quite proud of it until the sisters at the school cold-shaved the sinful thing off.

Precocious puberty is defined as the onset of puberty before age 7 or 8 in girls or age 9 in boys. There is a range, and this has been part of the problem of establishing the "normal" age of puberty. Girls might enter full-blown puberty anytime between ages 9 and 15 boys between 11 and 17.

Complicating matters further are racial differences. On average, African American girls show signs of puberty, with breast development and chemical changes in their bodies, almost two years sooner than white girls, at age 8.8. This is according to the largest study on precocious puberty, involving over 17,000 girls, by Marcia Hermann-Giddens of the University of North Carolina, published in 1997 in the journal Pediatrics. By age 8, nearly 50 percent of the black girls but only 15 percent of the white girls had begun pubertal development.

Also, historically, the age of first menstruation, called menarche, declined about 0.3 years per decade from the mid-1800s until the 1960s, attributed to better nutrition. Today the average age of menarche is 12.2 for African Americans and 12.7 for Caucasians.

In the milk?

Precocious puberty is more serious than the expense of needing to buy razors and tampons.

The early growth spurt can retard fuller growth in adolescence, as the brain tells the bones that growing time is over. Girls under age 10 aren't mentally prepared for monthly periods. And earlier sexual desires &mdashwith a mature body and immature mind&mdashcan lead to earlier sexual encounters.

When precocious puberty entered the radar screen in the early 1990s, the first suspects were hormones in milk and meats, particularly the artificial bovine growth hormone, rBGH. But this is a protein hormone, destroyed in human digestion, not a steroid hormone like estrogen.

Pollutants are a serious problem. Plastics and insecticides can break down into chemicals similar to estrogen. This is thought cause hermaphroditic fish. Ivelisse Colón of the University of Puerto Rico identified a compelling connection between exposure to chemicals called phthalates and a large increase in breast development among Puerto Rican girls younger than 7.

Kids too fat

Few scientists are ruling out the impact of pollutants. But the more logical cause of precocious puberty seems to be childhood obesity. Here we have a working theory plus numbers to back it up.

Puberty requires the body to have a certain weight and fat distribution, hence the delay for female gymnasts and ballerinas. So 8-year-old girls weighing as much as a normal 12-year-old are at risk for precocious puberty.

But also, across the board, fat children have high levels of the protein leptin. This chemical, through a complicated chain involving the hypothalamus and pituitary gland, can stimulate the release of the three main hormones in puberty: hypothalamic gonadotropin-releasing hormone, luteinizing hormone and follicle-stimulating hormone.

A study by Kirsten Krahnstoever Davison of Penn State, published in 2003 in Pediatrics, was one of several studies documenting that over 50 percent of overweight girls enter puberty early. Similarly, studies on young girls adopted from poor countries, suddenly introduced to high-fat diets and entering early puberty, reveal the fat cell connection.

While a cleaner Earth is a fine idea, the best prevention for precocious puberty seems to be to keep kids healthy and running wild like kids.

Christopher Wanjek is the author of the books &ldquoBad Medicine&rdquo and &ldquoFood At Work.&rdquo Got a question about Bad Medicine? Email Wanjek. If it&rsquos really bad, he just might answer it in a future column. Bad Medicine appears each Tuesday on LiveScience.

Your child may have a noticeable growth spurt because of their early puberty, but one of its complications is an abrupt and early end of that growth, leading to short stature as an adult.

“If this is a concern, please see your doctor,” Benjamin said.

While your child’s physical changes may be a concern, it’s also important to consider their psychological needs. Children with early puberty are more at risk for low self-esteem, depression, and substance abuse. Your son or daughter may require counseling to help address the changes in their body and the effects of any treatments they receive. Your doctor will help you access counseling services.

Why is puberty coming earlier for girls today?

Puberty is the time, usually between ages 10 and 12, when girls develop into women and boys transform into men. Most of us can remember puberty by its more awkward moments -- acne erupting all over our faces, hair sprouting in places we never expected, body odors we couldn't identify and feelings we had no idea what to do about.

Puberty was difficult enough at age 12, but imagine having to go through it at age 7 or 8. Today it's not that uncommon for a first grader to need a training bra, or a third grader to get her period. Starting the process of puberty earlier than normal is called precocious puberty, and it seems to happen more regularly these days, particularly in girls (precocious puberty is about 10 times more common in girls than in boys) [source: Carel et al.].

All of the physical and emotional changes that herald the onset of puberty in girls begin when an area of the brain called the hypothalamus secretes gonadotropin-releasing hormone (GnRH). This hormone signals the pea-sized pituitary gland at the base of the brain to release its own hormones -- luteinizing hormone (LH) and follicle-stimulating hormone (FSH) -- which stimulate the ovaries to produce estrogen. The release of this sex hormone leads to breast growth, body hair, acne, adult body odor and a girl's first menstrual period.

Sometimes this process doesn't start exactly as planned. In most cases of precocious puberty, the hypothalamus releases its hormone too early. This is called central precocious puberty. In other cases, a problem with the ovaries or adrenal glands leads to too much estrogen production. This is called peripheral precocious puberty.

­How early does puberty need to start to be considered precocious? Back in the 1960s, doctors considered 8 to 13 the normal age range for puberty to begin in girls. Then, as pediatricians started noticing the signs of puberty in greater numbers of their very young patients, the definition of precocious puberty started to shift.

In a landmark 1997 study of more than 17,000 girls, researchers discovered that nearly 15 percent of Caucasian girls and almost 50 percent of African-American girls had already started showing signs of puberty by age 8 [source: Herman-Giddens et al.]. This study led to new guidelines that consider precocious puberty to be breast or pubic hair development before age 7 in Caucasian girls and age 6 in African-American girls (however, this new guideline is controversial, and many pediatricians still stick to the previous recommendation of age 8 or younger) [source: Kaplowitz].

Why are girls starting puberty so early?

Female Puberty: What Causes Precocious Puberty?

In most girls who start puberty early, there is no obvious cause. The cycle of hormone production that starts with the ­hypothalamus just triggers sooner than normal.

In rare cases, precocious puberty can be caused by a physical problem, such as a brain tumor or injury to the brain or spinal cord, a genetic disease (such as McCune-Albright syndrome), an infection (meningitis or encephalitis), an abnormality in the brain (hydrocephalus) or cysts in the ovaries.

The condition might also have a genetic cause in a very small percentage of girls. Researchers discovered a mutant gene in one 8-year-old girl with precocious puberty, which they believe led to the surge of estrogen that triggered her puberty early [source: Teles et al.].

As to the reason why puberty appears to be coming earlier and earlier for young girls, researchers have several theories. Some have suggested that exposure to chemicals, including polychlorinated biphenyls (PCBs, which were once used as coolants and flame retardants) and phthalates (which are added to plastics to make them more flexible), may be partly to blame because these chemicals break down into substances that are similar to the hormone estrogen.

One study that followed 600 pregnant women and their children found that girls who were exposed to high levels of PCBs in the womb started puberty sooner than girls who didn't have such exposure. Researchers have also been curious about the rash of early puberty cases that hit Puerto Rico in the 1980s and '90s. Girls there started growing breasts as young as age 2. Researchers discovered that many of the Puerto Rican girls had high levels of phthalates in their bloodstream. A lot of the girls had also been fed soy formula as babies. Soy contains natural plant compounds called isoflavones, which can act like estrogen in the body [source: Lemorick].

In the 1990s, suspicion turned to hormones in milk and meat -- especially artificial bovine growth hormone. However, researchers say this hormone isn't a steroid like estrogen, and it's destroyed too quickly during digestion to have any real effect on the human reproductive system.

Right now, the biggest culprit seems to be the super-sized American diet. The rate of obesity in children ages 2 to 11 has nearly tripled over the last few decades [source: CDC]. Fat cells produce estrogen, as well as the hormone leptin, which can stimulate the release of the hormones that trigger puberty. Girls who are overweight also produce excess insulin (a hormone that helps the body use starches and sugars from food for energy), which can stimulate the ovaries and adrenal glands to release more sex hormones. A 2003 study found that nearly 60 percent of girls with precocious puberty were overweight [source: Davison et al.].

With puberty starting earlier, a lot of girls are becoming young women before their time, as you'll read in the next section.

Stages of Puberty for Girls

Thelarche: During this stage, a girl’s breasts begin to form. This starts with a small change called a breast bud. Breasts begin to form around age 11, although recent studies show that this process is now starting earlier. Girls may begin breast development around age 9. Sometimes only one breast will start to develop. Usually, within a few months, the other will start as well. This can be perfectly normal.

Pubarche: This is the initial appearance of pubic hair that is very fine. This usually happens around the same time as breast budding but may happen a little later.

Adrenarche: During this phase, pubic hair increases in amount and changes in texture from fine to coarse. This is also the time when underarm hair develops and body odor starts. During this phase is also when girls start to develop acne. This phase happens in the years between the first breast bud and the first period.

Menarche: This is the term used to describe the arrival of a girl’s first period. The average age is 12 years old, but a little earlier or a little later can be perfectly normal.

During puberty is also when a girl starts to get taller. Generally, after the appearance of breast buds, a girl’s height will begin to increase at a quicker pace than when she was younger. At some point during puberty, a girl will have a very rapid increase in her height, which is called a “growth spurt.” This typically happens several months before a girl’s period starts.

When a girl starts these changes and how long the process takes varies. On average, from the appearance of breast buds until the first period takes about 2 ½ to 3 years. But it is perfectly normal if it takes a little less time or a little more time to complete these changes.

Puberty is a part of normal growth and development. If puberty doesn’t happen or happens too early, it may be a sign that something may be wrong and needs medical attention. The following may indicate a problem and should be discussed with a doctor: