The growing rates of child obesity in the United States may be causing girls to reach puberty at an increasingly younger age, a study released Monday concluded.

After following 354 girls, University of Michigan researchers found those who were fatter at age 3 and continued to gain weight reached puberty, as defined by breast development, by age 9.

Earlier evidence has shown that girls are entering puberty at younger ages than they were 30 years ago.

During the same time period, child obesity rates have also skyrocketed.

"If you take a look at overweight children, obese children, females, there's no doubt from what we see in our practice, no doubt that they are prone to enter puberty at an earlier age," pediatrician Dr. Peter Nieman told CTV News.

The study found 168 girls had entered puberty by and age nine, with nearly two dozen developing their period by age 11.

"Our finding that increased body fatness is associated with the earlier onset of puberty provides additional evidence that growing rates of obesity among children in this country may be contributing to the trend of early maturation in girls," study lead author Dr. Joyce Lee said in a prepared statement.

In other words, the more fat cells the girl had -- at an age as young as three -- the more likely she was to enter puberty early.

The study did not, however, conclude why the link exists.

Some doctors speculate that a body-made protein is responsible.

"The link may be a protein called leptin, which is made in fat cells and seems to promote normal reproductive functions," Dr. Paul Kaplowitz told CTV News.

Past studies have examined the association between childhood weight gain and early puberty in girls, but until now there was no evidence on which factor caused the other.

"It was unclear whether puberty led to the weight gain or weight gain led to the earlier onset of puberty. Our study offers evidence that it is the latter," said Lee.

In addition to the many health issues associated with obesity, the study says reaching puberty at a younger age also prompts several social problems.

The study alluded to psychiatric disorders, drinking at a younger age and increased cases of sexual intercourse and teenage pregnancy.

The study measured obesity by a calculation of weight related to age and height to get a body mass index (BMI).

The girls who were defined as overweight had a BMI measurement of more than the 95th percentile.

The next stage for this type of developmental research is to discover whether puberty can be delayed by childhood exercise and diet.

"It's also important to determine whether weight control interventions at an early age have the potential to slow the progression of puberty," Lee said.

The complete findings are published in the March issue of Pediatrics.

With a report from CTV's Sarah Galashan