While most girls will get their first period at about age 12, the range can run from age 8-16. Most of these girls are "normal" by biological standards because puberty is a complex process of brain, body, and hormonal development. Still, there appears to be a trend toward precocious puberty (the clinical term for abnormally or unusually early puberty) in some boys and especially in girls, who are 10 times more likely to experience the condition.

Defining Normal Puberty

"Every kid is unique," says Robert Steiner, PhD, a neuroendocrinology researcher at the University of Washington. "Puberty is complex, affected by a constellation of hormonal factors. It depends on your ethnicity, your family history, and your nutritional status."

The onset of puberty is marked by the appearance of pubic hair and breast buds in girls. Next come breast growth and body shape changes in girls and testicular growth in boys; menstruation and voice changes come later. Menstruation occurs on average about two years after puberty begins.

Until recently, "normal" puberty was thought to begin between ages 8 and 14 in girls and between 9 and 12 in boys; however, recent research puts "normal" puberty as occurring as early as six in girls of certain ethnic backgrounds.

Rare Causes Of Early Puberty

In rare cases, especially when pubic hair is seen in children as young as five, precocious puberty is caused by tumors of the adrenal gland, ovaries, or brain, or by rare genetic disorders, like McCune-Albright Syndrome. While the appearance of pubic hair in such a young child warrants a visit to the pediatrician to rule out any serious causes, it may be just another variation on the norm.

Growth and Puberty

When puberty comes too early, it can affect growth. By late puberty, most children have reached 95% of their adult height. When puberty starts and ends too early, the sex hormones that help harden bones can end their growth too soon. Other problems may surface later in life. For example, the earlier a woman starts menstruating the more at risk she is for diseases like breast cancer.

If tumors or other serious causes of precocious puberty are ruled out and if a child's doctor believes the progress of puberty could harm her growth, hormone agonists, which cancel the hormonal domino effect, can delay puberty until a more optimal time in her growth. In some cases, experts believe that shifting the body mass downward (weight loss)—through better nutrition and increased activity—may also slow puberty's course.

The Mystery Of Puberty

Scientists and clinicians would like to clarify what is "normal" for the onset of puberty, but the issue is clouded by the controversy of what brings puberty on. The process is an intricate interplay of brain activity that leads to hormonal secretion. While experts can name some of the hormones involved in puberty, they don't know what the triggers are. While scientists have measured hormone patterns related to sleep (hormonal activity is high during sleep in pubescent children), they don't know why or how that affects growth or the development of sex characteristics.

Redefining Normal

Complicating the mystery is the controversy over whether there is a trend toward earlier puberty, particularly in girls. A number of clinicians in the United States now believe that some cases of early puberty are actually examples of using the wrong yardstick to measure.

In 1997, a review from the American Academy of Pediatrics’ (AAP) Pediatric Research in Office Settings (PROS) was published. Based on studies of more than 17,000 girls between the ages of 3 and 12, the PROS results showed that normal breast and pubic hair development occurred in American girls, particularly African American girls, much earlier than previous studies had shown.

The PROS study prompted the Drug and Therapeutics Committee of the Lawson Wilkins’ Pediatric Endocrine Society to endorse a revised definition of the onset of puberty (and hence, the definition of precocious puberty). Their statement lowered the normal age of puberty to 7 years in white girls and 6 years in African American girls.

Subsequently, Dr. Midyett and colleagues studied white girls aged 7-8 years and African American girls aged 6-8 years referred to their endocrine center for early onset of puberty. They found that 12% of these girls had pathologic causes for their sexual precocity. Interestingly, one-third of girls with two signs of puberty had advanced bone age.

Because these results were obtained from a referral group, they cannot be directly applied to the general population. However, these authors do raise the concern of missing significant pathology if the age range for normal puberty is changed. Dr. Chumlea and colleagues found that the age of onset of menstruation has not significantly changed since 1973. African American girls begin to menstruate earlier than white girls, with Mexican American girls somewhere in-between.

These seemingly disparate opinions underscore the need for continued research in this area. Until more is known, it may still be prudent to evaluate girls under the age of 8 with pubertal findings. The definition of precocious puberty in boys remains unchanged; thus, pubertal findings in a boy under the age of 9 merits evaluation.

Does Obesity Affect Puberty?

While many parents worry that toxins or food additives are a cause of early puberty, obesity may be a more likely culprit.

Since writing the article in the journal Pediatrics on behalf of the Lawson Wilkins’ Pediatric Endocrine Society, Dr. Kaplowitz and colleagues wrote a second article looking at the effect of increased body mass index (BMI) on earlier onset of puberty. They found that the BMI in pubertal 6-9 year old white girls was markedly higher than the BMI in prepubertal girls of the same age. A smaller difference was noted for African American girls.

"This is not revolutionary. We've known for years that very overweight kids tend to mature earlier," Kaplowitz says. "And we know that obesity is more of a problem in all age groups. But this provides additional evidence. And we know that kids with higher leptin levels (a hormone that appears to regulate obesity in rats) are maturing earlier."

The relationship between obesity and early puberty is not clear-cut, however. Multiple factors are likely involved in the development of precocious puberty. Studies are under way to look at these potential contributing factors.

The Role of Leptin

Researchers are still teasing out the relationship between leptin, a hormone produced by fat cells, and its decidedly important but unexplained role in launching puberty. The University of Washington's Dr. Steiner, a noted researcher on leptin and its neuroglandular role in puberty, believes it's a "permissive agent" but not a trigger in starting up puberty.

"Leptin was the first hormone that provided a link between weight and nutritional factors and reproduction," Steiner says. "The real challenge is finding what leptin is doing in the brain."