Puberty normally starts between ages 9 and 16; girls may not notice any physical changes until 11 to 16 years of age. Today, however, some girls are starting to see changes as early as age 6 or 7.

A research team backed by the Breast Cancer and the Environment Research Centers (BCERC) originally set out to more clearly categorize girls into pubertal maturity stages. What they found was that among roughly 1,200 girls aged 6-8, many showed early breast and pubic hair development.

Ten percent of white girls at age 7 had “breast development to some extreme”, as did 23 percent of black and 15 percent of Hispanic girls. These numbers jumped to 18, 43 and 31 percent for 8-year-old participants. Authors agreed that these rates were much higher than 10 and 30 years ago.

Possible explanations include increased exposure to hormones, such as estrogen. Another hypothesis is that environmental pollutants such as BPAs and parabens may be pushing young girls into puberty. Genetic mutations, endocrine disorders and other health concerns are also on the list, but researcher Dr. Abby Hollander of Washington University’s School of Medicine says we can expect to wait for any real answers.

Hollander and colleagues plan to further investigate ways the environment may affect the onset of puberty. Until then, read more from ABC News or see the study’s abstract in Pediatrics.