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  • Title: Differences between spontaneous and induced abortions as risk factors for breast cancer.
    Author: Zhang J.
    Journal: Epidemiology; 1996 May; 7(3):316-8. PubMed ID: 8728450.
    Abstract:
    Recent epidemiologic studies have found that spontaneous abortion has no association with breast cancer, while induced abortion is associated with a moderately increased risk. Although this finding may, in part, reflect confounding and ascertainment bias, there are credible biologic reasons for a difference in risk between these two events. Experiments with rats have indicated that proliferated but not differentiated terminal duct and lobular cells increase the susceptibility of breast tissue to carcinogens. Since ductal and lobuloalveolar breast cells proliferate in the first two trimesters of pregnancy and differentiate in the third, it would follow that the later (up through the late second trimester) the pregnancy is terminated, the greater the risk. Although there is a trend toward induced abortion at earlier gestational ages, spontaneous abortion tends to be the earlier event. Even when induced and spontaneous abortions occur at the same gestational age, dead embryos or fetuses are probably retained in utero for several weeks before expulsion, meaning hormone levels drop appreciably earlier than is the case with induced abortion, where the hormone profile drops only after termination. Moreover, since the average time to another pregnancy is significantly shorter after spontaneous abortion (12 months), the protective effect of subsequent full-term pregnancies may dilute the risk effect of spontaneous abortion to a greater extent than is the case with induced abortion. Now that most induced abortions occur in the first trimester, the implications for breast cancer risk should become minimal for both types of interrupted pregnancies.
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