Why is puberty starting younger?
Mona Chalabi
Yet another study has found that girls are starting puberty at a younger age, and that the trend is linked to obesity. But is precocious puberty the new norm?
The measurement of puberty has barely changed since 1970 when the seminal study by Marshall and Tanner was published. That might seem surprising given that it's generally considered one of the biggest milestones in every person's physical development.
What's normal?
Girls have always developed earlier than boys - on average the onset of puberty happens between the age of 8 and 12 years in girls and between 9 and 14 years in boys.
The duration of puberty varies too - in girls, puberty takes on average almost
3 years to complete (although anything between 2 and 6 years is still considered normal) while in boys, 4 years is normal.
The signs
There are three main developments that are tracked in the 'Tanner stages' (named after the British paediatrician that first researched them): for girls it's breast development, for boys it's genital development and for both sexes, pubic hair is a key indication that puberty is underway.
Other signs, like voice change, facial hair, growth spurts and menstruation might be important but they're not the key measures when it comes to monitoring the age of puberty.
The measurements can be quite specific - puberty has started in boys only once their testicles are greater than 3 millilitres. Tanner stage V, the last milestone, indicates that puberty has finished once a boy has a testicular volume greater than 20 ml and a penis of 15 cm in length (5.9 inches).
What is the new normal?
The study published today was based on 1,200 girls in America. The research focused solely on the development of breast tissue, partly because other studies have found that the average age of menstruation has remained quite stable. The change in puberty it uncovered wasn't enormous; it found that it was starting only a couple of months earlier than in the 1990s.
But in a wider historical context, the change appears far more stark: a study by Dr. Marcia Herman-Giddens found that
in 1860, the average age of the onset of puberty in girls was 16.6 years. In 1920, it was 14.6; in 1950, 13.1; 1980, 12.5; and in 2010, it had dropped to 10.5. Similar sets of figures have been reported for boys, albeit with a delay of around a year.
A big finding from today's study was the wide differences between ethnic groups. On average, African American girls started getting breasts at 8 years 10 months compared with 9 years 8 months in white and Asian girls, while for Hispanic girls it was around 9 years 4 months.
Those differences are important given that so little is known about worldwide trends.
Not just girls
Last year,
a study (also by the American Academy of Pediatrics) looked at more than 4,000 boys in 41 US states. Using the Tanner stages, it found that their testicle size indicated the start of puberty just before the age of 10. Previously, 11 and a half was generally seen as the age boys hit puberty. Like girls, the explanations put forward were focused on diet and exercise.
Obesity is consistently cited as a factor that affects the age of puberty.
International trends
The World Health Organisation, the go-to source for country comparisons on health data has very little about the onset of puberty. So, other sources have attempted to draw together different studies to observe international trends.
One study found that puberty was happening earlier across Europe, although country-level differences persisted.
Why?
Research constantly flags up the link between BMI (Body Mass Index, used to indicate if an individual is overweight or obese) and puberty. But there is more to it than that.
For example,
one study found that children who had been adopted recently from a developing county to Denmark were 10 to 20 times more likely to develop precocious puberty than children in the developed country. The study also found that the difference could not be explained entirely by changes in nutrition, body weight, or body fat. Similar findings were found in
studies of children immigrating to Sweden, France, Belgium, Switzerland and the United States.
Other factors that can influence biology relate to a child's environment - stress, climate, light cycles and chemical exposures all have established links, though their magnitude is still not entirely understood.
Key terms
- Thelarche (the-lar’ke): Onset of female breast development
- Adrenarche (ad‘ren-ar’ke): Onset of hormone-dependent signs of puberty (pubic hair, acne, and adult body odor)
- Pubarche (pu-bar’ke): Another term for adrenarche
- Menarche (me˘-nar’ke): Onset of menstruation
Update: what's the role of chemicals?
Several commenters below were interested in the link between chemicals and early onset puberty. For example…
We asked
Felicity Lawrence, our expert on the food business who pointed us towards some interesting research on the topic. Several studies focus on 'endocrine disruptors' - chemicals that can disrupt the hormones of mammals, humans included.
The language in this field is confusing to say the least - reading that "
phthalates may promote female puberty by increasing kisspeptin activity" or that
dichlorodiphenyltrichloroethane can have dangerous effects doesn't really help in understanding what the risks are.
Most of the acronyms refer to pesticides - or 'environmental chemical pollutants'. Phthalates meanwhile "are used as plastic softeners and as preservatives in some cosmetic products and they may be found in plastic toys, hair sprays, deodorants, shampoo, nail polish and perfumes".
It's important to remember that dose, duration and age at exposure all play a role, but there is mounting evidence that these chemicals may be a factor in early puberty.
There have been more animal than human studies on this, but the findings are stark. After a chemical spill of an insecticide called
DDT, one study found "male alligators living in the lake had smaller phalluses, reduced serum testosterone levels and abnormal gonadal structures" (
gonads are testes or ovaries).
So, why is puberty happening earlier? The Department of Environmental
Healthat Boston University sums it up neatly:
The causes of most precocious puberty cases remain unknown. While many of the cases are attributed to excess weight gain or (paradoxically) improved nutrition, exposures to phthalates and other endochrine-disrupting chemicals have also been implicated as possible risk factors. The limited data show that the worldwide incidence of precocious puberty has increased and the age of pubertal changes has decreased over the past two decades.
And why do people care so much? For one thing, early puberty in girls has been linked to breast cancer.
