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Many girls in the U.S. may be entering puberty at younger ages now than in previous decades, and obesity appears to be the major factor contributing to this shift, a new study finds.
The researchers looked at more than 1,200 girls ages 6 to 8 in the San Francisco area, the Cincinnati area and New York City, and examined them multiple times between 2004 and 2011. Entering puberty was defined as the age at which a girl's breasts started to develop.
The results showed that white girls entered puberty on average at 9.7 years old, which is three to four months younger than the average age reported by scientists in a 1997 study, and much younger than the average age suggested by data from the 1960s.
Black girls started puberty at 8.8 years old on average, which was not different from the results of the previous study. The average age of entering puberty for Hispanic girls was 9.3 years, and 9.7 years for Asian girls, the researchers found.
In line with previous studies, the researchers also found that obesity was strongly linked to early puberty, and girls with higher body mass indexes (BMIs) were more likely to have reached puberty at younger ages, according to the study published today (Nov. 4) in the journal Pediatrics.
"The influence of BMI on the age of puberty is now greater than the impact of race and ethnicity," said study researcher Dr. Frank Biro, a professor of pediatrics at Cincinnati Children's Hospital in Ohio.
"These girls were born and raised in the midst of an obesity epidemic, Biro told LiveScience. "This is yet another impact of obesity epidemic in this country."
Multiple studies, including some done in other countries, have reported children starting puberty earlier, the researchers said. For example, a 2009 Danish study reported girls experienced breast development nearly a year earlier than those born 15 to 16 years prior.
"With each new study in the past two decades, we hope the age of 'early puberty' has bottomed out," Marcia Herman-Giddens, a researcher at the University of North Carolina in Chapel Hill, wrote in an editorial published with the new study in Pediatrics.
"When each 'new study' has been published, however, we find the trend toward early puberty has continued," Herman-Giddens wrote.
Earlier development of breasts may not necessarily mean a shift in the age of the first menstrual period, the researchers noted. That has to be determined in future studies.
Several factors besides obesity may affect girls' development as well. Today’s children may be less active, and consume fewer fruits and vegetables than those born in the previous decades.
Diets high in meat and dairy products, along with high-stress families, hormone-laced hair products and insulin resistance are among other factors that could be leading to earlier puberty, Herman-Giddens said. [12 Worst Hormone-Disrupting Chemicals]
“Each individual girl is exposed to multiple factors in today’s environment, many not present decades ago, that may potentially influence her pubertal onset,” Herman-Giddens said.
However, it’s not clear how these factors interact to affect puberty, she said.
Regardless of the causes of the trend, earlier maturation may have long-term medical consequences, Biro said. Studies have suggested girls who mature earlier have a higher risk of developing high blood pressure and type 2 diabetes later in life.
Earlier maturation may have social consequences too, Biro said.
Biro said, "There's a greater mismatch now between how [girls] look, and how they behave and how they interact with other people."