Girls are entering puberty at ever younger ages. What are the causes, and should we be worried?
Marcia Herman-Giddens was a physician's associate in the 1980s when she noticed something strange. All her medical books pointed to the age of 11 as the time when girls would typically enter puberty — the transition to an adult body. But that wasn't what she was finding. Many girls in her clinic at the paediatrics department of Duke University Medical Center in Durham, North Carolina, had breast buds by the age of 9 or 10.
Determined to investigate, she put together a study1 of 17,000 girls. Her results confirmed a trend towards earlier puberty. “It was really the first study to call attention to this phenomenon,” says Louise Greenspan, a paediatric endocrinologist at the University of California, San Francisco. “Now other studies have corroborated it, including our work.”
The age of puberty, far from being a biological constant, has been changing for much of human history, and the clearest evidence is seen in women. The most extensive evidence comes from records of the age at first menstruation (the menarche), which occurs near the end of puberty. Menarche affected Palaeolithic girls between the ages of 7 and 13 (based on analysis of bone length, to indicate the amount of oestrogen exposure). It then shifted into the late teens as humans settled into agricultural societies. After about 1700, wherever sanitation, nutrition and infectious-disease control improved, girls started to menstruate earlier — about three-and-a-half months sooner per decade. Puberty in boys shows a similar trend. Trends for northern Europe2 show an average age of menarche of 16 years in the early 1850s; by the early twentieth century, it had dropped below 15; now it is 12-and-a-half (see 'Puberty trends').
The decline in menstrual age has slowed since the middle of the twentieth century in developed nations, but the start of puberty, marked in girls by the first signs of pubic hair and breast development, has continued to come earlier. Consequently, puberty now starts earlier and lasts longer. It typically begins at the age of 9 or 10, but sometimes as early as 6 or 7.
Adolescence researchers have been watching the modern trend of earlier onset with concern. “Early puberty is not good for kids for a variety of reasons,” says Laurence Steinberg of Temple University in Philadelphia. “In females, it's associated with an increase of certain types of cancer — breast and uterine, in particular.”
Investigators have also been exploring the reasons for the recent trend. The factors can vary — as they surely have over the centuries — but one possibility is the abundance of food. In animals, more access to food correlates with earlier puberty. This link and other findings raise the disturbing possibility that, to some extent, the decline in the age of puberty is a maladaptive response to the current obesity epidemic.
The rite of passage into adulthood begins silently in the brain with the release of the hormone kisspeptin, named for its discovery in Hershey, Pennsylvania, where The Hershey Company first manufactured its trademark chocolate kisses. Kisspeptin stimulates the neurons that trigger the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. At the same time, the adrenal glands, which are responsible for producing the sex hormones (androgens), become more active. These hormonal changes are followed physically in girls by the first signs of breast budding, and psychologically in the form of early romantic crushes.
Puberty progresses through the five Tanner stages of physical maturation, named after the mid-twentieth-century paediatrician James Tanner. In both sexes, pubic hair appears and then changes colour and distribution until it spreads to the thighs. At about the same time, girls' nipples become elevated. Menstruation typically begins in the girls' penultimate Tanner stage.
Normally, puberty in girls lasts for two to four years. However, Anastasios Papadimitriou, a paediatric endocrinologist at the University of Athens in Greece, points out that there seems to be some compensatory process that extends the maturation process in those who begin puberty early, causing puberty to last as long as six years.
At the population level, part of the explanation for earlier puberty is the increased survival of premature infants, who tend to go through puberty earlier. This early start seems to be related to a rapid 'catch-up' of postnatal growth, with premature infants having higher circulating levels of androgens in early childhood. But various effects that occur at an individual level also feed into the trend.
Obesity is the most reliably established of these. A landmark finding comes from a 2003 report3 from the ongoing Bogalusa Heart Study, a cross-sectional analysis of children in a rural community in Louisiana. The study examined more than 1,100 girls as 9-year-olds and again as 26-year-olds. Each standard-deviation increase in childhood body mass index at the age of 9 was found to double the girl's chances of beginning menstruation before the age of 12. The mechanism behind this effect seems to be related to leptin — a hormone produced by fat cells that inhibits eating by signalling satiety. Leptin also promotes the release of kisspeptin, so the more fat a girl has, the higher the levels of both leptin and kisspeptin — and the earlier she is likely to enter puberty.
Yet obesity alone does not explain all the instances of early puberty. There are several ways that modern life can take the brakes off development. Melatonin, a hormone that the body releases in darkness, is known to suppress kisspeptin. More exposure to light — including artificial light — keeps melatonin levels down and results in more kisspeptin, and so may promote earlier puberty. Children who live near the Equator receive the most natural light during childhood. As a result, they go through puberty earlier than children who live closer to the poles — even those with the same genetic background.
This effect can also be seen in other species. “If you went to a poultry farm and you looked at the building in which they were raising chickens, you would see that they keep the lights on a lot of the time,” says Steinberg. Chickens exposed to more light will reach sexual maturity more rapidly and start producing eggs earlier.
Another factor is the range of endocrine-disrupting chemicals in our environment. Flame retardants on fabrics, bisphenol A in plastics, and foodstuffs such as dairy, soya beans and red meat are on the list of suspects. Tobacco smoke, although not widely seen as an endocrine disruptor, may be having similar effects because of its cadmium content — prenatal exposure is associated with a lower age of menarche.
Psychology seems to have a role in early puberty as well. “There is a small effect of family conflict on accelerated puberty in girls,” says Steinberg. Factors such as a higher degree of mother–daughter affection, and more time spent by fathers in child care before kindergarten age, are linked to a later timing of puberty. By contrast, a 23-year longitudinal study4 found that girls who suffered sexual abuse from around the age of 7 or 8 went through puberty earlier than girls who did not. The mechanism is thought to be related to the physiological stress response, which affects hormone levels through the hypothalamic–pituitary–adrenal axis.
Early puberty is not evenly distributed throughout the population. Children from low-income families are more likely to reach puberty earlier than are children from wealthier ones. The probable reason is not hard to fathom — today's underprivileged groups are disproportionately affected by many of the factors implicated in early puberty. Children in poor families are more likely to be obese, have later bedtimes, have fragmented or chaotic families, and live in environments containing more of the chemicals that may disrupt hormones and increase the risk of obesity.
Parents generally do not need to worry if their children show signs of puberty at the early end of the normal range, around 8 or 9 years old, say Greenspan and Papadimitriou. In more extreme cases, counselling can prepare both the child and the parents for the years ahead. Clinicians will evaluate an early-developing child, primarily to rule out a brain tumour that may be disrupting the hormonal processes. Occasionally, if a child is abnormally precocious, parents can delay puberty by using GnRH agonists, which stop the pituitary from producing the hormones that trigger gonadal development. The GnRH agonists are injected or implanted in the skin, but they should not be used for too long, or at too late an age. “They may stop the healthy thickening of the bones that happens during puberty, and they may also change the brain's development,” says Greenspan.
Part of the counselling for girls includes a discussion of breast cancer. Many studies suggest a link between early menarche and breast-cancer risk in later life. A meta-analysis5 of 117 epidemiological studies found a 5% increase in breast-cancer risk for each year before the average start of puberty (13 years in this case). However, the risk might be even greater. Menarche is a memorable marker for puberty that is used in retrospective analyses. The onset of puberty, which starts with breast development, is harder to pin down but is probably more important. “It's the oestrogen exposure that matters,” says Greenspan. Counselling can help an early developing girl and her family learn how to minimize the child's risk of breast and uterine cancer.
The broad shifts in the age of puberty over the centuries suggests that there is no optimal age to start maturing. The healthy age range is broad, and the timing is affected by myriad factors, including individual differences. Ultimately, the most important factor for a girl's well-being may be social. “The best time for a girl to go through puberty is when her friends are doing the same,” says Papadimitriou. “That way she's no different from the others.”