Puberty comes sooner for overweight girls, according to a review of studies on the topic.

But the research also refutes the theory that girls who experience their first period at a relatively young age are predisposed to become obese as adults.

Instead, the study suggests that Childhood obesity helps drive both early puberty and adult weight troubles.

For parents concerned about the potential for Obesity in their daughter’s future, “the focus should be on the child being overweight rather than the timing of her first period,” said lead researcher Aviva Must, an associate professor of public health and family medicine at Tufts University School of Medicine in Boston.

She presented the findings, to be published in the September issue of Pediatrics, at a special “Back to School” press briefing on children’s health, held Thursday in New York City and sponsored by the American Medical Association.

According to Must, pediatricians have long noted a correlation between overweight and “early menarche” (first period) in young girls. “There are two competing theories as to why this happens,” she said.

One theory holds that excess body fat is a kind of reproductive signal that a girl is now healthy enough to sustain a pregnancy. The recent discovery of a fat-cell hormone called leptin “suggests a mechanism by which that might actually happen,” Must said.

The second theory rests on skeletal maturity. “We know that children who are overweight have advanced bone development - they grow faster in all ways, and they are usually taller than their non-overweight peers,” Must said. “That same sort of growth promotion could be linked to the early onset of the maturational change.”

Neither of these theories have yet been proven, however. Another lingering question has been the association between early menarche and later obesity in adulthood.

To help solve that issue, Must’s team analyzed data from the Newton Girls Study, which tracked 700 Boston-area girls, all of whom were first recruited in 1965. The Newton researchers studied the girls’ health and maturation from before their documented first period through to their 20th period.

“What we found was that maturational timing - onset of first period - was not an important factor for adult obesity, once we accounted for the earlier overweight [as children],” Must said. “So, it appears that the timing of menarche is a consequence, rather than a risk factor for adult overweight.”

In fact, women who were overweight before their first period were 7.7 times more likely to be overweight as adults as women who were not, the study found.

That means that parents of girls who undergo early menarche - at 10 or 11, rather than the U.S. average of 12.5 years of age, for example - shouldn’t necessarily worry that their daughters will be doomed to obesity in adulthood. Early menarche may “just be part of normal growth and development for her,” Must said. “It’s not a cause for concern nor does it put her at any excess risk of later overweight.”

Parents should be concerned about an overweight or obese child, however, since it appears that early excess weight gain is linked to adult obesity, Must said.

Earlier puberty is simply another consequence of that weight gain, she said, noting that the timing of a first period for American girls “is down by about 2.5 months from 25 or 30 years ago. We think that probably has to do with the epidemic of obesity in children - that’s what’s pushing it down.”

She said similar trends may be occurring in boys, although because sexual maturity in boys has no defining event such as menarche, “it’s been a lot harder to study.”