Trend of earlier puberty continues among US girls

The age at which girls in the U.S.A. reach puberty is continuing to dip, with heavier weights and changing national demographics playing important roles, according to a new study.

Research over the years has documented a gradual decline in the average age at which U.S. girls have their first menstrual period - from the age of 12.75 in the 1960s to about 12.5 in the early 1990s.

The new findings, published in the Journal of Pediatrics, show that the trend has continued. National data for the years 1999 through 2002 put the average age at menarche - the first menstrual period - at just over 12.3 years, researchers at Tufts University in Boston found.

Exactly why girls are reaching puberty earlier is unclear, but many experts find it concerning for a number of reasons. For one, early menarche is linked to an increased risk of breast and uterine cancers later in life, possibly due to the greater lifetime exposure to estrogen.

Then there are the questions about what’s causing the trend - including whether exposure to chemicals that act as hormone “disruptors,” such as PCBs and certain pesticides, is involved.

Researchers also suspect that the national jump in obesity and excess weight is playing a role, as it’s thought that body fat is important in triggering and maintaining a woman’s menstrual cycle.

The current study supports that idea, according to the authors, Sarah E. Anderson and Dr. Aviva Must.

Heavier girls, they found, tended to have their periods at a younger age, and the percentage of girls who were overweight or obese was higher in 1999-2002 compared with the years 1988-1994.

Through the early 1990s, 11 percent of girls ages 9 to 15 were overweight or obese, while 27 percent were “at risk” of becoming so. By 2002, nearly 17 percent of girls that age were overweight, and almost 33 percent were at risk.

But the study also found that shifting U.S. demographics may be playing in a role in the overall decline in girls’ age at menarche. When they looked at racial groups separately, the researchers found only very slight changes within groups.

For example, the average age at menarche among white girls dipped from 12.57 years to 12.52. And the decline among African-American girls was even smaller - from 12.09 to 12.06. Girls of Mexican ethnicity showed a larger change, but it was still not significant in statistical terms.

On the other hand, there was a clear downward shift in age at menarche in the heterogeneous ethnic group defined only as “other” - due to the small numbers of girls who did not fall into the three other categories.

This suggests that increases in these ethnic populations help account for the national decline in age at menarche, according to the researchers.

Still, Anderson and Must conclude, studies of national averages cannot address whether environmental factors, such as chemical exposures, are affecting individual girls’ maturation. More research is needed to answer that question, they say.

SOURCE: Journal of Pediatrics, December 2005.

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