Girls are beginning to grow breasts at an earlier age, and starting their periods sooner too, new research from Denmark shows.

The findings back up recent studies that found earlier breast development in American girls over the past several years, but still can't answer the question of why this might be happening, Dr. Lise Aksglaede of Rigshospitalet in Copenhagen, the lead researcher on the study, told Reuters Health. "At this point, we don't know what is happening, and that is also what worries us."

Aksglaede noted that she and her colleagues have seen an increasing number of girls with precocious puberty, meaning sexual maturation beginning before age eight. To investigate whether this might represent a trend, or simply greater recognition of the problem by parents, they looked at 1,100 girls who were studied in 1991-1993 and 995 examined between 2006-2008. The study participants ranged in age from 5.6 to 20 years old.

While the average age at which breast growth began was 10.88 years for the 1991 group, it was 9.86 for the 2006 group. Age at first menstruation was 13.42 for the 1991 group, and 13.13 for the 2006 group.

Most experts believe that the obesity epidemic may have something to do with earlier puberty in girls, Aksglaede noted, but she and her colleagues found no difference in the prevalence of overweight and obesity between the 1991 and 2006 groups. There also were no differences in levels of several reproductive hormones between the two groups, although the 8- to 10-year-olds tested in 2006 actually had lower estrogen levels than girls of the same age tested in 1991.

Some type of environmental factor, for example chemicals that have estrogen-like effects, may be responsible for the trend, Aksglaede said, but she pointed out that this is extremely difficult to study, given that there are so many different chemicals out there, and that levels girls are exposed to change constantly.

The health consequences of earlier puberty also aren't clear, the researcher said. While precocious puberty can have psychological consequences for girls, and may also stunt growth, the girls in the current study were still entering puberty at a relatively normal age.

"It is the first time we are seeing this in Europe," the researcher said. "It might be happening in other countries, but it hasn't been reported yet."