U.S. girls are entering puberty at a younger age than in the past. More than 10 percent of white 7-year-old girls have breast changes that signal the start of puberty, according to a study conducted in the mid-2000s. This is up from just 5 percent in the early 1990s.
The average age of a girl's first period also declined, from an average of 14 or 15 years in the past to about 12 years today.
"Experts aren't sure what's behind the increase in earlier puberty, but it's likely due to a combination of factors, including the childhood obesity epidemic and substances in the environment.
Early puberty in girls is a growing public health concern because studies have shown that girls who start puberty earlier are more likely to develop breast and uterine cancer later in life …
Early development in girls has been linked with poor self-esteem, eating disorders, and depression, as well as cigarette and alcohol use and earlier sexual activity."
As for what's causing the increasing in early puberty, chemicals in the environment, obesity, and above-average weight gain during infancy are all being considered as possible factors.