If you think that girls today are developing earlier than when you were growing up, you're right. Over the past decade, a number of studies have documented that the age of puberty in American girls has declined. Not only is early puberty associated with high-risk behaviors during adolescence (substance abuse and earlier sexual activity, for example), it may have long-term physical health implications as well.
"Early puberty increases a girl's risk of breast cancer later in life by widening her window of lifetime exposure to estrogen and lengthening that period of vulnerability between menarche and the first full-term pregnancy," said Jane Rizzo, RN, executive director of the Breast Cancer Fund, a national environmental health organization focused on preventing breast cancer and publisher of a new report, The Falling Age of Puberty in U.S. Girls: What We Know, What We Need to Know (available at http://www.breastcancerfund.org).
The author of the report, biologist Sandra Steingraber, PhD, adds, "When menarche occurs before age 12, breast cancer risk is 50% higher than when it takes place at 16. For each year menarche is delayed, the risk of breast cancer declines by 5% to 20%."
Thelarche (the start of breast development) and menarche are occurring increasingly early in the lives of American girls, but the age at which thelarche occurs is declining more rapidly, according to Steingraber, who is currently a scholar in residence at Ithaca College in New York. "Girls get their first periods, on average, a few months earlier than girls did 40 years ago, but they get their breasts, on average, one to two years earlier. Over the course of just a few decades, the childhoods of U.S. girls have been significantly shortened."
A 1997 study by Herman-Giddens and colleagues in Pediatrics reported the trend toward earlier puberty, using data derived from a cohort that involved 17,000 American girls. The analysis showed that at age three, 3% of black and 1% of white girls showed breast or pubic hair development or both. By age seven, those numbers rose to 27.2% and 6.7%, respectively. At age eight, virtually half (48.3%) of black girls and almost 15% of white girls showed some degree of development.
Steingraber's report notes that the mean age at menarche declined dramatically from the mid-19th century to the mid-20th century, and while the decline continues, it has slowed. The menstrual cycle begins in black girls at an average age of 12.1 years, while in white girls it's roughly 12.8 years. It remains unclear when racial differences first began to emerge, mostly because historical data in the United States concerned primarily white girls. Both black and Mexican American girls begin menstruating earlier than white, non-Hispanic girls; the mean age of thelarche is estimated to be 10 years in white girls and nine years in black girls.
The causes of the falling age of puberty aren't entirely clear, although evidence suggests that low birth weight and premature birth and environmental exposures to toxins that interfere with normal endocrine function appear to play roles, as does childhood obesity. A study published in the March 2007 issue of Pediatrics that followed 354 girls from a very young age (three years) demonstrated that increased body weight is linked to early puberty.
"We found that higher body mass index scores in girls as young as 36 months of age was associated with earlier puberty," said lead study author Joyce M. Lee, MD, MPH. "We also found an association among girls whose body mass index increases between 36 months of age and first grade."
Other factors that might play a role and are being studied include formula feeding in infancy, physical inactivity, and family dysfunction.
"With all of the various social, psychological, and environmental factors that can swing that window open early," says Rizzo, "precocious puberty is an excellent opportunity for us to work collaboratively, across different specialties: nutrition; psychology; and children's, women's, and environmental health. It's time to think differently about where and how risk starts-and to find public policy solutions that will protect our daughters now and later in life."
Based on what is already known, actions can be taken now, says Steingraber. "We can encourage exercise; we can improve children's access to healthy food, promote breastfeeding, and remove hormonally active chemicals from food and consumer products. These would all go a long way toward reversing the trend."
Roxanne Nelson, BSN, RN