Published: Aug 09, 2010 6:59 PM EDT
Updated: Aug 09, 2010 3:56 PM EDT

(CBS)  Girls in the U.S. are hitting puberty at younger ages, according to new research.

At age 7, about 10 percent of white girls and 23 percent of black girls had started developing breasts, compared to 5 percent of white girls and 15 percent of black girls in 1997, according to a study led by Dr. Frank Biro of Cincinnati Children's Hospital Medical Center. The results were published Monday in the medical journal Pediatrics.

Biro's team examined about 1,200 girls aged 7 and 8 in Cincinnati, New York and San Francisco.

At age 8, about 18 percent of white girls and 43 percent of black girls had entered puberty, compared to around 11 percent of white girls in 1997, but the same as black girls in that year.

Early puberty in girls is a concern because studies have shown they are more likely to develop breast and uterine cancer later - women who spend more of their lives menstruating have a higher risk of getting breast cancer.

Experts aren't sure what caused the earlier development of puberty.

But another study published Monday in Pediatrics shows that overweight girls are more likely to enter puberty earlier. The study was led by Dr. Mildred Maisonet from Emory University's Rollins School of Public Health and conducted on girls in Great Britain.

According to an article in Health.com, Biro said doctors are also worried about the psychological health of girls who hit early puberty. These girls have been linked to poor self-esteem, eating disorders, depression, as well as cigarette and alcohol use and earlier sexual activity.

"For the 11-year old that looks like she's 15 or 16, adults are going to interact with her like she's 15 or 16, but so are her peers," Biro said in the article. "It doesn't mean that they're psychologically or socially more mature."