DALLAS — Girls who are overweight beginning as early as age 3 are more likely to start puberty by the fourth grade, two years ahead of the American average, researchers say.

Girls who are heavy for their size at age 3 and whose weight-to-height ratio increases faster than normal between age 3 and first grade, are likelier to start puberty earlier than their peers, say researchers at the University of Michigan’s C.S. Mott Children’s Hospital. Their study appeared yesterday in the journal Pediatrics.

“This offers support that the obesity epidemic is probably contributing to this earlier age of onset of puberty in U.S. girls,” Joyce Lee, lead author of the study and a pediatric endocrinologist at C.S. Mott Children’s Hospital in Ann Arbor, said in an interview.

The study tracked 354 girls in 10 states from before puberty started. Early puberty has been blamed for higher rates of behavioral problems and stress, and earlier initiation into alcohol use and sexual intercourse, as well as adult obesity and reproductive cancer.

In the study, 68% of those who were overweight or at risk for being overweight began puberty at age 9. About 40% of the girls who weren’t overweight began showing signs of puberty by age 9.

Just as in the population at large, 30% of the girls in the group were overweight or at risk of becoming so.

The girls were tracked between age 3 and age 12, during laboratory visits at 36 months, 54 months, and grades one, four, five, and six.

Ms. Lee said her study has strengthened the evidence linking obesity to early puberty because it began tracking girls at a younger age than previous studies, had a larger sample size and greater geographic diversity.

The study used body mass index, or BMI, to categorize which girls fell into the weight-risk group. BMI is a number calculated from a person’s weight and height to calculate body fat.

Girls were considered to be entering puberty if their breasts had begun to develop or they had had their first menstrual cycle.