Does pregnancy protect against ovarian cancer?

A clear inverse relation between average completed family size and mortality from ovarian cancer in different populations of women is seen from one country to another; for successive generations of women living within the same country; in married and single women; and in different social, religious, immigrant, and ethnic groups. The findings suggest that pregnancy--or some component of the child-bearing process--protects directly against ovarian cancer. This protection seems to persist throughout life. The more-than-twofold increase in the age-standardised ovarian-cancer death-rate in England and Wales since 1931 can be explained largely by changes in the average completed family size.