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For many years there has been some concern that the hormones used in oral contraceptives might contribute to an increase in the risk of breast cancer. Research has failed to confirm an overall association between birth control pills and breast cancer. However, in recent years it has become clear that, for specific subgroups of women, oral contraceptives do impose an increase in risk. These subgroups include women who started using oral contraceptives at an early age and used them for an extended period, women who used birth control pills for an extended period prior to the birth of their first child, and childless women who have used oral contraceptives for an extended period. Research has also demonstrated that women who developed premenopausal breast cancer after using oral contraceptives at an early age tended to have larger tumors at diagnosis and were more likely to have lymph node metastases. These observations suggest that oral contraceptive use is associated with more aggressive cancers. For this reason, investigators examined some of the cellular characteristics of breast cancers in a group of 175 women with premenopausal breast cancer to determine if features thought to be associated with greater cancer aggressiveness differed in the women with a history of oral contraceptive use. Measuring the S-phase fraction, which is the fraction of cells undergoing DNA synthesis at a given time, it was found that women who used oral contraceptives before the age of 20 tended to be higher in this measure of cell proliferation activity. This increase in S-phase fraction was evident in aneuploid tumors, which have an abnormal chromosome complement, but not in diploid tumors. Furthermore, aneuploid tumors were also more common in this group. It was also found that the S-phase fraction was 49 percent higher among women who had early abortions. Previous research has shown that abortions and miscarriages increase the risk of breast cancer; the present research shows that they also are associated with markers of cancers of increased aggressiveness. While the precise mechanisms by which oral contraceptive use or abortions affect the development of breast cancer are not known, one possibility is that since the normal proliferation of breast tissue is greatest in the teenage years, factors which influence breast tissue during this time may result in more aggressive tumors. (Consumer Summary produced by Reliance Medical Information, Inc.)