“What we need to ask is, Why has BMI gone up?” he says. “Decreased physical activity and a more calorically dense diet are probably part of the puzzle. But I think another critical piece is our ubiquitous environmental exposure to endocrine-disrupting chemicals,” or EDCs. This class of chemicals (including phthalates, bisphenol A and others) is used in many consumer products (shower curtains, plastic bottles, couch cushions) and has been shown to mimic estrogen and other naturally occurring hormones in the human body.

Biro theorizes that some of these chemicals may promote weight gain or contribute to early puberty by influencing how cells and the body regulate metabolism, which then affects estrogen production. He is currently leading a study tracking the growth and development of 379 girls from age six onward that has been examining relations between their pubertal development and environmental exposures. Trauma could be another explanation: “Stress can also change your estrogen levels,” Biro says.

Source: Why Are Girls Getting Their Periods So Young? – Scientific American