Women who eat a fatty diet when pregnant could unwittingly be increasing the risk of their daughters and granddaughters getting breast cancer, research has shown.
Chronic exposure to hormone-like chemicals in food and water may have a similar effect, evidence suggests.
The findings, from a study of rats, could explain why breast cancer often runs in families even though known inherited genes account for only a small number of cases.
Researchers fed pregnant female rats a high-fat diet from before conception to the time they gave birth. Their daughters were 55% to 60% more likely to develop breast cancer than animals whose mothers had a normal diet.
When the daughters eventually gave birth themselves, their female offspring were equally at greater risk of breast cancer.
The raised risk from fatty diets did not extend to the great granddaughters of the original rats. But the impact of a hormone supplement was felt across three generations.
Pregnant rats given synthetic oestrogen in their food had daughters with a 50pc higher chance of breast cancer. The same level of increased risk affected both their granddaughters and great-granddaughters.
Chemicals with oestrogenic properties are abundant in plastics, paints, food packaging and electrical equipment.
Though they are present at low non-toxic levels, experts believe long-term exposure to such "endocrine disrupting chemicals" could have important biological effects on fertility and other aspects of health.
Dr Sonia de Assis, from Georgetown University in Washington, US, said: "This study suggests directions for future research in women.
"Could a woman's susceptibility to breast cancer development be determined by what her grandmother ate when she was pregnant, or if she was exposed to high levels of oestrogen -- perhaps unwittingly -- through the environment?
The findings, published in the latest online edition of the journal 'Nature Communications', are believed to result from environmental effects on genes.
Exposure to certain chemicals in the womb can determine whether specific genes are switched on or off.
The evidence suggests that such "epigenetic" effects can become cemented into DNA and passed on to future generations.