Overweight or obese girls get their first period months earlier than their normal-weight peers.
It's nothing new that girls are getting younger and younger when they have their first period, 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 Sarah Keim, a researcher who wasn't involved in the new study.
About 17% of American kids and teens are obese
, according to the Centers for Disease Control and Prevention.
For the study, researchers used information on body mass index (BMI) - a measure of weight in relation to height - and age at first period from about 3,200 Danish girls born between 1984 and 1987.
The girls started their period just after they had turned 13, on average, which is about half a year later than in the U.S. Keim said part of the reason for this difference may be that African-Americans tend to start their periods before white girls.
On average, a girl got her period about 25 days earlier for every point her BMI increased. For a female of about average height and weight, a one-point change in BMI is equivalent to about six pounds.
Overweight and obese girls, for example, got their period three to five months before normal-weight girls, said Anshu Shrestha, a graduate student at UCLA School of Public Health, who worked on the study.
There has been past research showing a link between BMI and when girls start menstruating. However, since this study was done more recently, it shows that the link is holding up in today's generation, 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 BMI when pregnant went up, the girl's period came about a week earlier, according to the new study, which was published in the journal Fertility and Sterility.
Keim said the Danish findings reinforce the importance of keeping a healthy weight.
"It's important for your entire life, starting from very early on, and it can even affect your children's health."Do you think BMI matters when it comes to menstruation?