These findings are extremely worrying but women shouldn't assume that their childbearing choices will automatically lead to a diagnosis

Delyth Morgan, Breast Cancer Breakthrough

The findings are published in the British Journal of Cancer on Wednesday.

Compared to those who gave birth at 22, women who had their first child in their 30s were 63% more likely to develop breast cancer before the menopause - and 35% more likely to get the disease afterwards.

The study also found that the later a girl started her periods, the lower her chance of developing breast cancer later in life.

A woman who began menstruating at 15 had only two-thirds of the risk of premenopausal breast cancer compared to one who started at 11.

Researcher Dr Francoise Clavel-Chapelon said: "Our study has provided all sorts of information about the influence of reproductive and hormonal factors on the development of breast cancer.

"This information will help us to understand the mechanisms by which breast cancer develops.

Anxiety

"It's especially interesting that the influences on a woman's risk of breast cancer can be so different before and after she reaches the menopause."

The number of women developing breast cancer has increased in the UK in recent years.

The reasons for this are not known, although some suspect that the more common decision of many women to delay starting a family until their 30s may be at least contributory.

The average age at which women in England and Wales have their first child is now 27 years - compared to 24.5 years in 1980.

In the past, the fear of breast cancer has added to the anxiety already felt by women who have miscarried

Dr Francoise Clavel-Chapelon, researcher

The number of births per 1000 women in the 35 to 39 age group has soared from 31.1 in 1990 to 40.5 in 2000.

The chief executive of Breast Cancer Breakthrough, Delyth Morgan, said: "These findings are extremely worrying but women shouldn't assume that their childbearing choices will automatically lead to a diagnosis, or not, of breast cancer in later life.

"Having children early is no cast-iron guarantee that you will not get breast cancer. Similarly a late pregnancy doesn't mean that you will experience the disease."

Reassurance

However, the figures on miscarriage are reassuring - the researchers found no association between having miscarriages and developing breast cancer either before or after the menopause.

Dr Chavel-Chapelon said: "In the past, the fear of breast cancer has added to the anxiety already felt by women who have miscarried.

"I'm very glad to be able to allay those fears."

Professor Gordon McVie, the joint director-general of the new charity Cancer Research UK, said: "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.

"Only by looking at very large numbers of women, as this study has, can we start to build up a picture of how and why breast cancer develops."