Declining ovarian cancer rates in U.S. women in relation to parity and oral contraceptive use.

Ovarian cancer incidence and mortality rates have declined among U.S. women age 35-59 years during the period 1970-1995. Epidemiologic studies have shown that ovarian cancer risk decreases with increasing parity and increasing duration of oral contraceptive use. During this period, parity has declined while oral contraceptive use has increased. We compared temporal trends in observed ovarian cancer incidence rates with rates predicted by changes in parity and duration of oral contraceptive use to determine whether the changes in these characteristics could explain the declining rates in younger women. In addition, we wished to examine whether oral contraceptive use continues to be protective to postmenopausal women. To predict changes in rates between 1970 and 1995, we assumed that increases in parity and duration of oral contraceptive use induce proportional decreases in incidence rates. We found that the rates predicted by these assumptions agreed well with observed rates in young women (age 30-49) but were substantially lower than observed rates in older women (age 50-64). The data indicate that the relative decrease in incidence rates due to the protective effect of oral contraceptive use declines with age.