MINNEAPOLIS (WCCO) — It is a phenomenon that doctors don’t fully understand: More girls beginning puberty at a younger age.

Most parents want their children to hold onto their childhood as long as possible, but when a girl starts developing in second and third grades, the changes that come with puberty can be confusing — even devastating psychologically.

A study published in the journal Pediatrics found that by age seven, 23 percent of black girls, 15 percent of Hispanic girls, 10 percent of white girls and 2 percent of Asian girls had started developing breasts.

On a gorgeous spring day at Como Park, families soaked in the weather, but when we brought up girls going through early puberty there was immediate recognition.

Melissa Miles has twin daughters.

“I find it alarming,” she said. “When I was younger, it seemed like it happened a lot later than it does now.”

Dr. Bradley Miller is a pediatric endocrinologist with the University of Minnesota. He says one reason for the growing trend childhood obesity.

“Fat tissue actually makes more estrogen,” said Miller.

However, slender girls can also develop early. Some researchers point to environmental concerns like BPA that has been widely used in items like plastic bottles and cups growth hormones in foods like milk. Dr. Miller says the research is not conclusive, however.

“Some studies have supported that and other haven’t, so I don’t know if we know a cause,” said Miller.

The early onset of puberty means girls will be of shorter stature and at greater risk for medical diseases, like breast cancer. But the psychological effects can be the most serious.

At St. Paul Academy and Summit School, school counselor Sara Mairs says children need to talk about the changes.

“My biggest concern would be about the girl who would internalize and feel bad about who she is and start to engage in more destructive behaviors,” said Mairs.

Miller says parents should see a doctor if the signs of puberty emerge before age eight.

This is a phenomenon that is unique to girls. While it is possible for boys to go through puberty earlier than their peers, it is far more unusual.