Twice as many white girls in the US reach puberty aged 7 as a decade ago. No one is sure why this should be the case, but obesity and exposure to chemicals that mimic the female hormone oestrogen are the prime suspects.

The figures come from a study of 1200 girls in three US cities. Of the girls studied, 10.4 per cent of white 7-year-olds had breast development consistent with the onset of puberty, compared to 5 per cent in a 1997 study.

Earlier studies had identified early puberty mainly in black girls.

Marcia Herman-Giddens of the University of North Carolina at Chapel Hill, who led the 1997 study, expressed shock at the increases.


“To find the girls are starting breast development earlier and earlier is extremely concerning,” she says. “To have that much change in such a short time, it has to be the environment.”

It appears that the proportion of black 8-year-olds entering puberty has dropped from 48 per cent in 1997 to 43 per cent today. Frank Biro of the Cincinnati Children’s Hospital Medical Center in Ohio, head of the new study, suggests this might be a sign that the numbers of black girls experiencing early puberty may finally be levelling off, while the percentage of white girls affected is still rising.

“Part of it is the increase in overweight and obese girls,” says Biro. Hormones released by the extra fat cells could play a role. His team is also checking blood and urine samples for chemicals that mimic oestrogen, from plastics in the environment, for example, to soya in the diet.

Journal Reference:Pediatrics, DOI: 10.1542/peds.2009-3079