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Description of Evidence

    Incidence of Outcomes Per 1,000 Women continued...

    Factors of Unproven or Disproven Association

    Abortion

    Abortion has been suggested as a cause of subsequent breast cancer. Studies showing an association used recalled information in populations in which induced abortion had a social or religious stigma, differential reporting of prior abortion by breast cancer patients, and controls. Trials conducted in social environments where abortion is accepted, however, have not shown an association with breast cancer.[116,117,118,119,120,121]

    A meta-analysis of women from 53 studies in 16 countries with liberal abortion laws was performed.[122] Analyses were performed separately on 44,000 women with breast cancer who had information on abortion collected prospectively (i.e., 13 studies) versus 39,000 women with breast cancer from whom information was collected retrospectively (i.e., 40 studies). The RR of breast cancer for women with spontaneous abortion was 0.98 (95% CI, 0.92–1.04 for those with prospective data collection and 0.94–1.02 for retrospective data). The RR after induced abortion was 0.93 (95% CI, 0.89–0.96; P = .0002) if the information was collected prospectively but was 1.11 (95% CI, 1.06–1.16) if it was collected retrospectively. Additional analyses of the number and timing of aborted pregnancies were performed, but none showed a significant association with breast cancer.[122]

    Oral contraceptives

    Oral contraceptives have been associated with a small increased risk of breast cancer in current users that diminishes over time.[123] A well-conducted case-control study did not observe an association between breast cancer risk and oral contraceptive use for every use, duration of use, or recency of use.[124]

    Another case-control study found no increased risk of breast cancer associated with the use of injectable or implantable progestin-only contraceptives in women aged 35 to 64 years.[125]

    Environmental factors

    Whether occupational, environmental, or chemical exposures have an effect on breast cancer risk is controversial. Although some findings suggest that organochlorine exposures, such as those associated with insecticides, might be associated with an increase in breast cancer risk,[126,127] other case-control and nested case-control studies do not.[128,129,130,131,132,133] Studies reporting positive associations have been inconsistent in the identification of responsible organochlorines. Some of these substances have weak estrogenic effects, but their effect on breast cancer risk remains unproven. The use of dichloro-diphenyl-trichloroethane was banned in the United States in 1972, and the production of polychlorinated biphenyls was stopped in 1977.

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