Nearly twice as many young women are being diagnosed with breast cancer that has spread to other parts of their bodies compared to 30 years ago, say scientists.
A study found that the metastatic breast cancer rate rose around two per cent each year between 1976 and 2009 among women aged 25 to 39.
This is the most dangerous kind, with fewer than one-third of women surviving at least five years after diagnosis.
Young women only undergo mammograms if they have a family history of breast cancer
However, the overall rate of cancers in that group is still small according to the researchers at the University of Washington.
Their data reveals one in 173 women will develop breast cancer before she turns 40, and she is more likely to have a worse prognosis than older sufferers.
Study leader, Dr Rebecca Johnson, said the increased number of cases of the aggressive form of the disease could be linked to the rising rate of obesity as women eat more and exercise less.
She added that hormonal birth control may play a role.
Dr Johnson said: 'We think that the likelihood is that since this change has been so marked over just a couple of decades, that it's something external, a modifiable lifestyle-related risk factor or perhaps an environmental toxic exposure, but we don't know what.'
She added that more research is also needed into the potential effects of hormones in meat or plastic in bottles.
The team analysed data from cancer registries run by the National Cancer Institute. As expected, they found that the number of early breast cancer diagnoses increased among middle-aged and older women during the study period, likely due to widespread screening.
They also found that cancer incidence among younger women rose from one in 65,000 in 1976 to one in 34,000 in 2009.
Human metastatic breast cancer in the lymph nodes. This is the most aggressive type of the condition
More of the increase appeared to be in cancers that are sensitive to estrogen.
This is 'comparatively fortunate,' the authors noted in the Journal of the American Medical Association, because those cancers are somewhat more responsive to treatment and have longer average survival rates in general.
However, surgeon Julie Margenthaler, who was not involved in the study, told Reuters Health that the new study was limited by a lack of data on women's family history, including which ones were carriers of BRCA gene mutations.
'It is intriguing data, but I think that it's going to have to be validated in some other datasets,' she said.
Read more: http://www.dailymail.co.uk/health/article-2285299/Number-young-women-diagnosed-advanced-breast-cancer-nearly-doubles-30-years.html#ixzz2MDPiWzGa
Follow us: @MailOnline on Twitter | DailyMail on Facebook