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| (Girls can start their periods as young as 10) |
The team scanned the genetic code of more than 100,000 women, reporting their findings in the journal Nature Genetics.
In the UK, girls as young as 10 are now showing the first signs of puberty.
A specialist said early puberty was linked to an increased risk of female cancers later in life.
The reasons why girls are going through puberty several years earlier than a century ago are not well understood by scientists.
Some have suggested a relationship between early puberty and obesity, and the latest research, carried out by the Reprogen consortium of scientists from the US, Europe and Australia, supports this idea.
Among the 30 genes highlighted by their genome research were some already linked to fat metabolism and weight regulation.
However, it is still not clear whether being obese or overweight in childhood is itself the cause of early puberty, or just another consequence of a different mechanism.
In addition, the study does not show how much of the risk is due to genes, and how much other environmental factors such as diet and upbringing are responsible.
Fat stores
One author, Dr Ken Ong, from the Medical Research Council Epidemiology Unit in Cambridge, said: "We know that girls who are overweight are more likely to go through puberty at young ages.
"If rates of childhood obesity continue to rise we will see many more girls with puberty at young ages."
Dr Anna Murray, another of the researchers from Exeter University, suggested that holding larger stores of fat might signal to the brain that it had the resources to start a woman's reproductive life.
She said: "We found that the timing of puberty is related to fatty acid metabolic pathways - there is evidence that the brain can sense these types of body fats."
Aside from the confusion experienced by young children entering puberty, it carries other health risks in the longer term.
Professor Anthony Swerdlow, from the Institute of Cancer Research, said that early puberty meant a higher risk of female cancers, particularly breast cancer.
This was possibly due to a higher lifetime exposure to sex hormones such as oestrogen.
He said that good diet and exercise, or the lack of it, in early childhood might be crucial to health many decades later.
He said: "What we increasingly believe is that, at least for breast cancer, the risk starts very, very young, even before puberty.
"If you are to prevent it, you need to be aware that you may be laying down your future risk from childhood."
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