One of the largest studies on the relationship between having children and not developing breast cancer (Francoise Clavel-Chapelon et al.) concluded that:

• the average lifetime risk of developing breast cancer had been 9%

• women who had borne a child before they were 22 had been the least likely to develop the disease

• compared to these, women who had had their first child in their 30s were 63% more likely to develop the disease pre-menopause, and 35% more likely post-menopause

• the protective effect of having several children was only evident post-menopause

• having a miscarriage had not, contrary to the findings of earlier smaller studies, increased the risk

• women who had begun to menstruate at 15 were two thirds less likely to develop the disease than those who had begun at the age of twelve

In the third term of pregnancy the breast cells change and prepare the milk-producing tissue. British Journal of Cancer editor Professor Robin Weiss commented that the study suggested that, if the breasts were able to develop into their lactating state shortly after adolescence, this might prevent the development of breast cancer.

Francoise's study analysed the birthing histories of 91,260 women aged 40-65, then followed their health outcomes for ten years.