Like many 10-year-old girls, Lindsay Kendrick likes to play sports and attend camp during the summer.
And like a growing number of girls her age, Lindsay also hit early puberty very early. Lindsay's mother, Beth, said her daughter first started menstruating when she was only 9.
"I thought it was going to happen early," said Beth. "She's been one of the tallest in her class, even taller than a lot of boys, and she started having early breast development."
Kendrick took Lindsay to her pediatrician, who told Beth that early puberty is much more common now, and said that Lindsay's period would probably start sometime in the next year.
"She started about two months after that," Beth said.
It's been a challenge for both Lindsay and Beth.
"She doesn't like talking to any of her friends about it," Beth said. "I try to keep up with her to make sure I send products with her to school."
A new study published in the journal Pediatrics may help lay the groundwork for future research that determines why, as past research has shown, more girls may be entering puberty early.
In the study, researchers assessed more than 1,200 6- to 8-year-old girls in three metropolitan areas for breast development and the appearance of pubic hair, both signs of early puberty. The research showed that by using these methods, scientists may be able to determine more accurately what factors could lead to early puberty in girls.
One of the study's authors told ABC News' Senior Health and Medical Editor Dr. Richard Besser, however, that determining early puberty was not the focus of the research.
"I don't think from this study we can say the age is going down in the world at large," said Mary Wolff, who is a professor of community and preventive medicine at Mount Sinai School of Medicine. "This study was not designed to look at if puberty was happening early or not."
The authors point out that the study does not use a nationally representative sample of subjects, and does not look at development over time to account for environmental exposure, dietary differences or other factors related to race and ethnicity. Additionally, some subjects were selected because they had existing risks for early puberty.
Another important element missing from this study is information about the onset of menstruation, which could indicate whether puberty has actually started.
"It's going to take a lot of follow-up to say whether this is really puberty," said Dr. Abby Hollander, associate professor of pediatrics at Washington University School of Medicine in St. Louis. "Within five years, we should be able to say whether the average age girls get their periods is earlier."
But pediatricians said past research suggests puberty may, indeed, be striking earlier than the 10-to 14-year-old age range that the National Institutes of Health set as the norm for girls.
"Ten percent of white girls at age 7 have breast development to some extreme, which is way younger than our original standard of evaluating normal versus abnormal," said Dr. Ann Budzak, a pediatrician with Gundersen Lutheran Health System in LaCrosse, Wisconsin.
Past research has shown a steady decrease in the age that children -- specifically girls -- are hitting puberty. Evidence first surfaced in a study published in the journal Pediatrics in April 1997. In that study, researchers found that the average age for breast development or the growth of pubic hair was 9.7 years old for Caucasian girls and 8.1 years old for African-American girls. A number of subsequent studies have shown similar results -- that girls are "growing up" at younger and younger ages.
"This study adds to the literature in terms of looking at whether puberty is happening early so we can figure out what's causing it," said Dr. Elizabeth Alderman, professor of clinical pediatrics at the Albert Einstein School of Medicine.
So far, there's been no clear, singular explanation as to exactly why this is happening. One theory suggests girls' exposure to estrogens and other organic pollutants in the environment could be providing the early trigger that sets puberty into motion.
There are hypotheses that bisphenol A, or BPA in the environment could be triggering early puberty. Other environmental toxins which are believed to have an impact on pubertal age are parabens -- preservatives found in shampoos and other cosmetic products -- and phyoestrogens found in plants.
"None of these possible triggers have been studied adquately," said Budzak.
The authors of the current study concur. They wrote "[v]ariations in the timing of pubertal maturation may be sensitive 'sensors' of the effects of environmental exposure in human populations," and are in fact in the first year of a study looking whether environmental factors cause early puberty.
Another theory places blame on the growing epidemic of obesity among children because fat tissue is known to produce estrogen. But that theory raises questions of its own.
"If puberty happens earlier because girls are heavier, and gaining that weight is sending hormonal signals to start puberty earlier, is that really normal, or a sign of obesity, which is abnormal?" said Hollander.
One 2007 study seemed to suggest that a stressful family environment may cause puberty to kick in early for young girls. And in 2008, yet another study found a gene mutation that could be responsible for a small number of these cases.
Occasionally, precocious puberty can be the result of a tumor or other problem in the brain, or a brain injury or infection; these cases, however, are rare.
There are also endocrine disorders that can cause early puberty, including thyroid disease, polycystic ovarian syndrome and others.
"These are the types of things to screen for when a little girltarget="external" seems to be showing signs of early puberty," said Budzak.
Experts have conflicting opinions about whether or not this study will redefine what is normal in terms of early puberty.
According to Budzak, a new definition of normal can have huge treatment and evaluation implications. "If a girl is in this new range of normal, the child doesn't need to go through all the tests to evaluate something that is no longer abnormal," she said.
"It shows that it could be totally normal for an 8- or 9-year-old to have breast development," said Alderman.
Experts do agree that finding the cause for early puberty is important because of potential health concerns and developmental issues in the future.
"Epidemiological studies have shown that women with breast cancer have a younger age of menarche [the start of menstruation]," said Alderman.
"Other studies have linked early puberty to mental health issues and emotional issues," said Budzak. "They may enter into social relationships with older peers that may not be appropriate for girls their age."
Early puberty may also hit parents hard, too, as Beth Kendrick can attest.
"I was more upset than she was," said Kendrick. "She grew up so fast, and I'm also worried about all that responsibility for a girl her age."