Scientists have identified mutations in a gene responsible for some cases of early puberty, an issue of growing concern to doctors, as children begin developing earlier and earlier.
Doctors define early puberty as the development of secondary sex characteristics before age 8 in girls and before age 9 in boys.
It's no longer rare to see children developing so young.
About 15% of 7-year-old girls show the beginnings of breast development, according to a 2010 study in the journal Pediatrics. In African-American girls, that rate is nearly one in four.
While parents may worry about the emotional toll caused by early puberty, doctors say they're concerned because early puberty increases a girl's later risk of breast cancer, endometrial cancer, heart disease, diabetes and obesity.
Even more disturbingly, doctors say, the trend appears to be accelerating.
Between 1991 and 2006, the median age of girls' breast development fell by one year, from age 11 to age 10, according to a large Danish study in Pediatrics. There's less evidence that boys are entering puberty at earlier ages, although one study last year suggested boys were maturing six months to two years earlier than other studies have shown.
In the new study, published in Wednesday's New England Journal of Medicine, researchers focused only on cases in which early puberty runs in the family. They found mutations in a gene called MKRN3 in five of 15 families studied.
Because researchers focused on so-called familial cases, the new study doesn't explain all examples of early puberty, or why the phenomenon is increasing, says study co-author Ursula Kaiser, chief of endocrinology at Brigham and Women's Hospital in Boston.
Her study focused on just 40 people from 15 families in which early puberty runs in the family.
Five of the families had mutations in a gene called MKRN3, which normally acts as a brake to keep children from entering puberty. Mutations in that gene may allow puberty to begin too soon.
That gene may be one of many that affect the timing of sexual development, says Paul Kaplowitz, chief of endocrinology at Children's National Medical Center in Washington.
Because genetic change occurs slowly over time, he said that lifestyle or environmental factors likely interact with genes to affect the age of puberty.
Many suspect that the rising obesity rate has played a large role, Kaplowitz says. Being overweight increases a girl's risk of early puberty. Scientists also are investigating environmental causes, including chemicals in pesticides and plastics that can act like hormones.
Most cases of early puberty don't need to be treated, Kaplowitz says. And in some cases, parents and pediatricians may mistake body fat for breast development.
When doctors feel treatment is needed, hormone therapy can help slow the process so that kids mature at a more typical age, Kaplowitz says.