CHICAGO — When it comes to the birds and the bees, some parents may want to have that talk with their boys a little sooner than they expected.

Researchers have found signs of puberty in American boys up to two years earlier than previously reported — age 9 on average for blacks, 10 for whites and Hispanics. Other studies have suggested that girls, too, are entering puberty younger.

Why is this happening? Theories range from higher levels of obesity and inactivity to chemicals in food and water, all of which might interfere with normal hormone production. But those are just theories, and they remain unproven.

Doctors say earlier puberty is not necessarily cause for concern. And some experts question whether the trend is even real.

Dr. William Adelman, an adolescent medicine specialist in the Baltimore area, says the new research is the first to find early, strong physical evidence that boys are maturing earlier. But he added that the study still isn't proof and said it raises a lot of questions.

Earlier research based on 20-year-old national data also suggested a trend toward early puberty in boys, but it was based on less rigorous information. The new study involved testes measurements in more than 4,000 boys. Enlargement of testes is generally the earliest sign of puberty in boys.

The study was published online Saturday in Pediatrics to coincide with the American Academy of Pediatrics' national conference in New Orleans.

Dr. Neerav Desai, an adolescent medicine specialist at Vanderbilt University in Nashville, said he's seen a subtle trend toward slightly earlier puberty in boys. He said it's important for parents and doctors to be aware so they can help children emotionally prepare for the changes that come with puberty.

Doctors generally consider puberty early if it begins before age 8 in girls and before age 9 in boys.

Boys are more likely than girls to have an underlying physical cause for early puberty. But it's likely that most, if not all, of the boys in the study were free of any conditions that might explain the results, said lead author Marcia Herman-Giddens, a researcher at the University of North Carolina in Chapel Hill.

Problems such as thyroid abnormalities and brain tumors have been linked to early puberty. But boys with chronic medical conditions or who were using medicines that could affect puberty were excluded from the research.

In girls, early puberty has been linked with increased chances for developing breast cancer, but whether it poses health risks for boys is uncertain. Some scientists think early testes development may increase the risk for testicular cancer, but a recent research analysis found no such link.

"If it's true that boys are starting puberty younger, it's not clear that means anything negative or has any implications for long-term," said Adelman, a member of the American Academy of Pediatrics' committee on adolescence.

For the new study, researchers recruited pediatricians in 41 states who participate in the academy's office-based research network. Doctors asked parents and boys aged 6 to 16 to take part during regular checkups. The visits took place between 2005 and 2010.

Half of the boys were white. The rest were almost evenly divided among blacks and Hispanics.

On average, white boys started puberty at age 10, a year and a half earlier than what has long been considered the normal average. For black boys, the average age of 9 was about two years earlier than in previous research. Among Hispanics, age 10 was similar to previous research that only involved Mexican-American boys. The new study included boys from other Hispanic backgrounds.

Testes enlargement was seen at age 6 in 9 percent of white boys, almost 20 percent of blacks and 7 percent of Hispanics.

Pubic hair growth, another early sign of puberty, started about a year after testes enlargement in all groups but still earlier than previously thought.

In girls, breast development is the first sign, and recent research suggested that it starts at age 7 in about 10 percent of white girls, 23 percent of blacks and 15 percent of Hispanics. That's substantially higher than rates reported more than a decade ago.

But some experts have questioned methods used in studies in girls, noting that the age when girls start menstruating has not changed much and remains around age 12 on average.

Dr. Dianne Deplewski, a pediatric endocrinologist at the University of Chicago, has not seen any increase in boys referred to her for signs of early puberty. She said it's possible that the new study results were skewed by families who brought their boys to the doctor because they already had concerns about their health.

The study had other limitations. Testes were measured just once, and doctors weren't randomly recruited but volunteered to participate. That means it's possible that those with early maturing patients were overly represented, but Herman-Giddens said it's unlikely boys in the study were different from those in the general U.S. population.

She said the research methods weren't perfect but that they're the best to date. She also stressed that the results shouldn't be used to establish a "new normal" for the start of puberty in boys.

"Just because this is happening doesn't mean this is normal or healthy," the researcher said.