The idea of a reaching puberty between ages 10 and 13 is no longer a reality for millions of American girls, says a new study published in the Pediatrics journal. Nowadays, parents and doctors are seeing girls reaching puberty as young as 7 or 8 years old. Breast growth, menstruation and sexual impulses triggered by hormones, are just a few of the issues parents are now discussing with their pre-teen girls, reports the New York Times. The study's authors attribute factors like obesity and environmental chemicals to the increased rates. Girls are now exposed to an abundance of food, and increased body fat (leading to obesity) produces sex hormones. Though it hasn't been proven yet, experts also wonder whether exposure to environmental chemicals like plastic, which mimics estrogen in the body, speeds up puberty. Doctors also worry that too much exposure to estrogen increases breast cancer risk among girls. Study author Dr. Frank Biro suggests we start "living green." "It may help for families eat together and to consume well-balanced diets. Regular physical activity may help, too."