Overweight or obese girls get their first period months earlier than their normal-weight peers, according to a Danish study.
It's nothing new that age at menarche has been falling, but experts worry that the current obesity epidemic could be fuelling that trend.
Early-onset menstruation is linked to later health problems such as breast cancer, said Dr Sarah Keim, a researcher at The Ohio State University College of Medicine in Columbus who wasn't involved in the new study.
Girls who start menstruating earlier are also more likely to have sex before most of their peers do, Dr Keim added, which increases the risk of teen pregnancy and sexually transmitted diseases.
For the study, researchers analysed information on body mass index (BMI) and age at first period from about 3,200 Danish girls born between 1984 and 1987.
On average, the girls had their first periods just after turning 13, which is about half a year later than in the US Dr Keim said part of the reason for this difference may be that African-Americans tend to start their periods before white girls.
On average, menarche came 25 days earlier for every point of BMI above average. Overweight and obese girls, for example, got their period three to five months before normal-weight girls, said Anshu Shrestha, a graduate student at the University of California, Los Angeles School of Public Health who worked on the study.
Past research has shown a link between BMI and the onset of menstruation. Since this study is recent, it shows the link persists in today's generation, Dr Keim said.
The researchers also found that a girl's mother's weight was related to when her daughter started menstruating, but less so than earlier work had hinted.
For every point her mother's pre-pregnancy BMI went up, the girl's period came about a week earlier, according to the new study, which appeared in Fertility and Sterility.
(Reuters Health, Leigh Krietsch Boerner, March 2011)
Genes, menstruation and menopause