WASHINGTON (AP) - Parents come to Dr. Gilbert August mystified: Their little girls, around age 8 and sometimes younger, already are showing unmistakable signs of puberty.

"They say, 'My God, she's too young, do something,'" August says.

Endocrinologists say they're seeing more girls with precocious sexual development, and some medical studies suggest the baffling trend is real, that the beginning signs of puberty are showing up earlier in today's girls than their mothers.

How early? At age 8, almost half of black girls and 15 percent of white girls start developing breasts or pubic hair. At 9, those numbers reach 77 percent of black girls and a third of whites.

More striking, 27 percent of black girls and 7 percent of whites develop these early puberty signs at age 7 - the second grade - according to a landmark puberty study.

Until recently, doctors and parents didn't expect to see budding breasts until around age 10.

Nobody knows what's causing the shift. Fat is the leading theory, because childhood obesity has doubled in the last 20 years and body fat certainly can spur hormones. Some scientists are hunting environmental culprits, and point to a small study from Puerto Rico - where early breast development is such a problem that it can begin at the stunning age of 2 - that casts suspicion on certain chemicals in cosmetics and plastics.

Nor can anyone explain the racial differences, or whether puberty is shifting for boys, who just now are being studied.

Regardless of the questions, is earlier onset of puberty harmful? In rare cases, a cyst or tumor may cause extremely early puberty, so doctors always check. Super-early puberty also can steal some growth, leaving girls a little shorter than they otherwise would be.

But as younger puberty becomes more the norm, the bigger worry is how girls still at the age of Saturday morning cartoons handle the physical changes, especially when their playmates remain flat-chested or they attract unwanted attention from older boys.

"From a psychosocial standpoint, you have a child who looks sexually mature at an age where they can't make judgments associated with their physical appearance. That's what really worries the family," explains August, a pediatric endocrinologist in Washington, D.C.

Shots of a medicine called Lupron can stall puberty. But unless girls are extremely young or have other medical problems, counseling or support groups to help explain puberty and deal with shyness, teasing or sexual overtures often are more recommended.

The good news: Age of menstruation, which dropped from about age 17 in the 19th century to between ages 12 and 13 by about 1960, is holding steady.

But Dr. Marcia Herman-Giddens of the University of North Carolina sparked concern with her 1997 landmark study of 17,000 girls that found outward signs of puberty that precede menstruation - budding breasts and pubic hair - were hitting younger. Now scientists are trying to unravel why.

Fat is the top theory. The fatter you are, the more your body can convert adrenal hormones into the female sex hormone estrogen. Overweight children's blood harbors more insulin, which also influences maturation. Scientists even are studying whether the protein leptin, produced by fat cells, influences glands that produce sex-related hormones.

"It does not mean that every overweight girl is going to enter puberty earlier, but on average they do," says Herman-Giddens, a child health specialist.

Related to the fat theory is concern that too many girls are couch potatoes. Regular, active exercise can delay puberty - think gymnasts and ballet dancers. Some research even suggests low birth weight is a factor, that the body may be overcompensating for undernourishment in the womb.

More controversial is environmentalists' concern that fetal exposure to certain chemicals, particularly byproducts of plastics and cosmetics called phthalates, could disrupt girls' normal hormone function. They cite a study in Puerto Rico, which has bafflingly high rates of early breast development, that found higher phthalate levels in the blood of 41 early-developing girls - some as young as 2 - than in 35 normal girls.

That's not proof phthalates are to blame; the cosmetics and plastics industry vehemently defend the chemicals as safe. But independent scientists say more study is needed.

Copyright 2001 The Associated Press. All rights reserved.