BREAST CANCER IN RELATION TO PATTERNS OF ORAL CONTRACEPTIVE USE

A total of 112 white female residents of King County, Washington, aged 35–54 years, who had received a first diagnosis of invasive breast cancer between July 1977 and August 1978, were interviewed concerning prior use of oral contraceptives. Their responses were compared with those of a random sample of 469 demographlcaily comparable women from the same popuiation. Overall, oral contraceptive use in the cases and controls was similar. However, use of oral contraceptives in preparous women was more common among cases than controls, with the estimated risk of breast cancer associated with such use being 2.2 times that of nonusers (90% confidence interval = 1.1–4.6). This relationship could be explained only in part by the effect of oral contraceptives in postponing or preventing childbirth. The association of breast cancer with use of oral contraceptives prior to ever giving birth has been observed in three studies, including this one, suggesting that the susceptibility of breast tissue to hormonal factors that influence the development of malignancy may be altered by having been exposed to the events of pregnancy.