The belief that girls who start menstruating before age 12 will be overweight later in life is unfounded, according to a study released yesterday.

Excess body fat has been found to jump-start puberty, with some larger girls starting their periods before age 12. But some doctors believed that the reverse could also be true -- perhaps girls who matured at an earlier age faced a higher risk of becoming overweight later.

That belief led some doctors to focus on delaying puberty as a way to combat obesity, said researchers from Tufts University School of Medicine in Boston.

"For the parents of a girl who is not overweight and who gets her first period early, it doesn't mean she's at increased risk for being overweight as an adult," said Aviva Must, the study's lead author.

"These findings are significant because they show us where our efforts should focus: childhood obesity," Must said.

Overweight girls can be expected to mature earlier, grow taller and develop breasts earlier than leaner girls, which should not cause concern, the study said. But they do have a higher risk of remaining overweight into adulthood, Must said.

The report was based on data from the Newton Girls Study, which followed 700 girls from the small Massachusetts city beginning in 1965. Researchers contacted about 450 of the participants when they reached an average age of 42.

The study will be published in the September issue of Pediatrics.