Women who start their periods early, have no children, or bear children late, are at increased risk of breast cancer, a study involving 91,000 French women has confirmed.
The research, the largest of its kind, looked at women between the ages 40 and 65. Most were teachers.
It suggested that women who had their first child in their thirties were 63% more likely to develop the disease before the menopause than those who gave birth before they were 22. They were also 35% more likely to get the disease after menopause.
A woman whose menstrual cycle began at the age of 15 was placed at only two-thirds the risk of premenopausal breast cancer of someone whose periods had started at 11.
There was a decrease in risk of 7% for each year the onset of periods was delayed. And the risk of postmenopausal breast cancer fell by 3% each year periods were delayed, according to researchers at the Institut Gustave-Roussy, Paris.
The study, based on questionnaires returned by the women from 1990-97, also suggests that the risk of breast cancer occurring early or before menopause is cut by 2% for each full-term pregnancy achieved. And for breast cancers occurring after the menopause the risk falls by 9% for each pregnancy.
Miscarriages, in some studies blamed for increasing risk of breast cancer, appear to be ruled out in this study as an important factor. But the study confirmed other long suspected links between reproductive and hormonal history and breast cancer.
The study is published in the British Journal of Cancer, whose editor, Robin Weiss, cautions against raising fears among career women who delay having children.
"You are not certain to get breast cancer because you don't have children," he states. He points to the lifetime risk of developing breast cancer, which is estimated at one in nine - and the rate is very low in young women, rising to one in 23 for those up to 60 and one in 10 up to age 85. The differences in risk revealed by the new study had to be placed in context, he said.
The French research, which included 1,718 women diagnosed with breast cancer during the study, divided the women into premenopausal and postmenopausal groups. The information gained is considered robust because so many were teachers and gave such detailed accounts of the history of their health.
Francoise Clavel-Chapelon, author of the study, said: "This will help us to understand the mechanisms by which breast cancer develops. It is especially interesting that the influences on a woman's risk of breast cancer can be so different before and after she reaches the menopause."
She was glad to be able to allay fears of breast cancer that added to anxiety already felt by those who had miscarried.
Gordon McVie, joint director general of Cancer Research UK, said: "The link between reproductive factors, fluctuations in hormones and women's breast cancer risk is extremely complex, and previous small studies have often produced confusing and conflicting results. Only by looking at very large numbers of women, as this study has, can we start to build a picture of how and why this cancer develops."
Professor Weiss, of University College London, a biologist, suggested the reduced risk in young women who had children might arise from the fact that breast cells, there to produce milk, had completed their natural function fairly soon after periods started.
Breast cancer is now the most common cancer in Britain, but threequarters of those patients diagnosed survive at least five years. Nearly 40,000 cases are diagnosed each year, and about four in five are postmenopausal women.
