Some people have speculated that chemicals used in plastics may contribute to the phenomenon of early puberty.

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Female Puberty: What Causes Precocious Puberty?

In most girls who start puberty early, there is no obvious cause. The cycle of hormone production that starts with the ­hypothalamus just triggers sooner than normal.

In rare cases, precocious puberty can be caused by a physical problem, such as a brain tumor or injury to the brain or spinal cord, a genetic disease (such as McCune-Albright syndrome), an infection (meningitis or encephalitis), an abnormality in the brain (hydrocephalus) or cysts in the ovaries.

The condition might also have a genetic cause in a very small percentage of girls. Researchers discovered a mutant gene in one 8-year-old girl with precocious puberty, which they believe led to the surge of estrogen that triggered her puberty early [source: Teles et al.].

As to the reason why puberty appears to be coming earlier and earlier for young girls, researchers have several theories. Some have suggested that exposure to chemicals, including polychlorinated biphenyls (PCBs, which were once used as coolants and flame retardants) and phthalates (which are added to plastics to make them more flexible), may be partly to blame because these chemicals break down into substances that are similar to the hormone estrogen.

One study that followed 600 pregnant women and their children found that girls who were exposed to high levels of PCBs in the womb started puberty sooner than girls who didn't have such exposure. Researchers have also been curious about the rash of early puberty cases that hit Puerto Rico in the 1980s and '90s. Girls there started growing breasts as young as age 2. Researchers discovered that many of the Puerto Rican girls had high levels of phthalates in their bloodstream. A lot of the girls had also been fed soy formula as babies. Soy contains natural plant compounds called isoflavones, which can act like estrogen in the body [source: Lemorick].

In the 1990s, suspicion turned to hormones in milk and meat -- especially artificial bovine growth hormone. However, researchers say this hormone isn't a steroid like estrogen, and it's destroyed too quickly during digestion to have any real effect on the human reproductive system.

Right now, the biggest culprit seems to be the super-sized American diet. The rate of obesity in children ages 2 to 11 has nearly tripled over the last few decades [source: CDC]. Fat cells produce estrogen, as well as the hormone leptin, which can stimulate the release of the hormones that trigger puberty. Girls who are overweight also produce excess insulin (a hormone that helps the body use starches and sugars from food for energy), which can stimulate the ovaries and adrenal glands to release more sex hormones. A 2003 study found that nearly 60 percent of girls with precocious puberty were overweight [source: Davison et al.].

With puberty starting earlier, a lot of girls are becoming young women before their time, as you'll read in the next section.