Changing patterns of breast cancer among American women.

Although overall mortality from breast cancer has changed little over time in the United States, age-specific rates showed distinctive patterns during 1950-75, with declines among premenopausal women, a rise then fall among perimenopausal women, and level then increasing rates among postmenopausal women. The trends appear related to the changing patterns of childbearing among young adult women over the first two-thirds of this century. The national mortality and birth data presented are consistent with existing analytic evidence of a protective influence upon breast cancer of early first birth, and suggest that the protection may be expressed at all ages above 30.