By Caroline Ryan
BBC News Online health staff in Copenhagen
Mothers of children who have cancer are more likely to develop breast cancer themselves, researchers have found.
Events during pregnancy may be key
They said the younger the child was when they were diagnosed, the higher the mother's risk.
Women's risk was also increased if they were the mothers of boys who had cancer.
Cancer Research UK scientists told the European Cancer Conference in Copenhagen, Denmark, they believed the increased risk was due to mother-child interaction during pregnancy, and said hormones may play a role.
Doctors from the Paediatric and Familial Cancer Research Group at the Royal Manchester Children's Hospital studied mothers of 2,604 children who had been diagnosed with cancers including lymphatic, brain, liver and kidney under the age of 15.
They examined data from the children's cancer register in the north west of England, and looked at children diagnosed with cancer between 1954 and 2000.
The women were aged from under 20 to 50 when their children were born. Researchers looked at what the expected rate of cancers would be or women in that age group, and compared that to the incidence in the group they studied.
They found that there were 95 cases of breast cancer in the group overall, a third higher than the expected number of 73.
In women whose children were diagnosed before they were five-and-a-half, there were 51 cases of breast cancer, compared with an expected 34 cases.
In mothers of boys diagnosed with cancer, there were 64 cases when only 40 were expected.
The majority of cancers occurred in pre-menopausal women.
The risk appeared to be higher in the early years of the child's life.
Professor Jillian Birch, director of the group, told BBC News Online: "We think that our findings indicate that there's something going on during pregnancy that's influencing a mother's risk of breast cancer.
"These mothers are unusually young to develop breast cancer."
She said that genetic mutations in both the mother and child, and hormonal changes during pregnancy, were both likely to be factors in the increased risk.
Mutations in one gene, tumour suppressor gene p53, have been identified as increasing the cancer risk in both young women and children, but Professor Birch said other genes were likely to be involved.
"Our hypothesis is that genetic mutation is present in both the mother and the child, but that the hormonal exchange during pregnancy affects the mother's cancer risk."
Professor Birch added: "If women are concerned, they should talk to their child's consultant or to their GP, and they can almost certainly get a referral to a clinical geneticist if they are really worried."