Women's risk of getting breast cancer increases if they have few children, start to have children late, or if their periods start early, a large scale French study has confirmed.
The study, published this week on the website of the British Journal of Cancer (http://www.bjcancer.comwww.bjcancer.com), followed nearly 100 000 French women over a 10 year period. They were aged 40-65 at the start of the study. All were with a national health insurance scheme and were mainly teachers. The women were sent follow up questionnaires every two years about their lifestyle, including reproductive factors, body build, smoking, medical history, and any family history of cancer.
The study also distinguishes between the effect on premenopausal and postmenopausal women.
Of the 91 000 women in the study, 1718 were diagnosed with breast cancer over the time period; at the end of the follow up 39 148 women were premenopausal and 50 996 were postmenopausal.
The research showed that women who had had their first child in their 30s were 63% more likely to develop breast cancer before the menopause than those who had given birth before the age of 22.
The risk difference was lower, however, for developing breast cancer after the menopause: women who had had their first child in their 30s were 35% more likely to develop breast cancer after the menopause than those who had given birth before the age of 22.
It also showed that each full term pregnancy reduced the risk of breast cancer by 8%.
Early onset of periods also increased a woman's risk of developing breast cancer. Women who had started their periods at 15 were at only two thirds the risk of premenopausal breast cancer compared with women whose periods had started at 11. The risk decreased by 7% for every year that periods were delayed.
Dr Françoise Clavel-Chapelon, who undertook the study, said the information would help them understand the mechanisms which allowed breast cancer to develop.
"It's especially interesting that the influences on a woman's risk of breast cancer can be so different before and after she reaches menopause," she said.
The study also found that there was no evidence that a history of miscarriage put women at higher risk of developing breast cancer either premenopausally or postmenopausally.
Professor Gordon McVie, joint director general of Cancer Research UK, welcomed this large scale research: "The link between reproductive factors, fluctuation in hormones, and women's breast cancer risk is extremely complex, and previous small scale studies have often produced confusing and conflicting results," he said.
The study is ongoing and will be looking at the effect of diet and hormone replacement therapy in the future.