New research adds further evidence that girls are entering puberty at younger and younger ages, with implications for their physical and mental health.
By 8-years-old, more than 1-in-10 girls have already begun developing breasts, which marks the technical start of puberty for girls, according to a new study published Monday in the journal Pediatrics.
The findings varied by race. Among 7-year-olds, about 10% of whites, 15% of Hispanics and 23% of blacks have some breast tissue. Among 8-year-olds, the numbers grew to 18% of whites, almost a third of Hispanics and half of blacks.
The researchers, from the University of Cincinnati College of Medicine, Mount Sinai School of Medicine in New York and Kaiser Permanente in San Francisco, were surprised by how early the onset of puberty was in the study, which looked at 1,239 girls.
The findings track a trend identified by a number of studies, including a Danish study published last fall that found the average age of breast development fell by about a year compared with girls born 15 years earlier. The trend appears in girls in many Western countries.
Research conducted in past decades put the average age at between 10 and 11 years.
"We need to understand better all the factors that are contributing to earlier maturation," said Frank Biro, director of adolescent medicine at the Cincinnati Children's Hospital Medical Center and first author on the study, which was funded by several federal grants.
Doctors are concerned because early onset of puberty is associated with physical consequences, such as a greater risk of breast cancer, and psychological ones, such as lower self-esteem and poorer body image, due in part to increased attention and teasing from peers. Girls who mature early are also more likely to be treated as if they are older than they are, either sexually or emotionally, and may be unable to deal with the demands, said Dr. Biro.
Studies looking at breast cancer, such as a large study of more than 100,000 women published in the American Journal of Epidemiology in 1998, have shown associations between fewer menstrual cycles and a reduced risk of breast cancer, presumably because of less exposure to the hormone estrogen. Women who had their first menstrual cycle before 12 years old have a slightly increased risk of breast cancer, according to the National Cancer Institute.
Estrogen facilitates pubertal development and is necessary to the menstrual process. There is less evidence to suggest that the average age of menarche has come down, but earlier pubertal onset means that girls' lifetime risk of exposure to estrogen is increased.
One of the main contributors to early puberty is thought to be increasing body weight and obesity rates. Environmental factors may also play a role and need to be further researched, say some experts.
Little is known definitively about any possible impact of environmental chemicals and hormones that girls may be exposed to in foods. But Dr. Biro's study will continue following the girls to look at development over time to try to figure out what effect chemicals such as phthalates, which are present in some plastics, plant hormones and pesticides may have on the body's endocrine system, which is responsible for manufacturing hormones. Several studies, including Dr. Biro's, will measure girls' exposure to various chemicals to investigate whether such contact does predict earlier maturation.
Fat cells produce hormones, and once a critical mass of fat tissue is reached, the hormone leptin is released to trigger puberty, according to JoAnn Manson, an endocrinologist and chief of preventive medication at Harvard-affiliated Brigham and Women's Hospital in Boston, who wasn't involved in the study.
A separate study published in Monday's Pediatrics found that higher weight gain during infancy was one factor related to early start of puberty.
Susan Nunez, chairwoman of the pediatric endocrinology committee of the American Association of Clinical Endocrinologists, noted the study's small numbers, which she said made it difficult to draw conclusions about the actual age of onset of breast development. The unequal representation of different ethnic groups, such as Asians, could affect the results, she added.
But other medical experts said that regardless of the exact percentages, there is a general consensus within the medical community that the age of pubertal onset is decreasing.
"There should be some sort of rethinking about chronological age versus pubertal age with public-health issues" such as when children are taught about their bodies or sex, said Elizabeth Susman, a professor of biobehavioral health at Pennsylvania State University.
Darrell Wilson, chief of pediatric endocrinology and diabetes at Stanford University's Lucile Packard Children's Hospital, said it isn't uncommon in his clinical experience to see girls as young as 8-years-old who have begun puberty. He cautioned parents against being overly concerned about the negative associations with early puberty, though those that do develop early would be examined as part of the normal course of follow-up.
However, if a child shows signs of breast development younger than age 5, it is often a sign that something is wrong. Puberty in these cases could be caused by something like an ovarian or brain tumor, he said.
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