Review on January 21, 2010
Induced abortion has long been linked to breast cancer risk in case-control studies. A full-term pregnancy in females prior to the age of 35 lowers overall risk of breast cancer, possibly by speeding up breast-cell differentiation. It's possible that a shortened pregnancy may not ensure sufficient differentiation to offset elevated levels of pregnancy hormones that may advance proliferation. Though, it's also possible that an incomplete pregnancy may not influence breast cancer risk at all and most epidemiologic reviews of spontaneous abortion (miscarriage) and breast cancer have not suggested a link. The investigation detailed below, the NHSII, examines relationships between lifestyle factors, reproductive factors (such as menopause status), and the incidence of breast cancer.
In the NHSII study Hazard Ratios (HRs) were altered for age, menopause status (pre-menopause or post-menopause), age at menopause, and post-menopause hormones use, among other things. Russo and Russo reported that, stopping pregnancy in rats canceled the protection against mammary tumorigenesis that pregnancy allows. They proposed that during the first three months of pregnancy the differentiation process may not be enough to offset the effects of increased levels of pregnancy hormones such as estrogen and progesterone that boost breast-cell division.
The NHSII investigation did not establish a relationship between induced abortion and breast cancer occurrence nor between spontaneous abortion and breast cancer incidence during a 10-year follow-up. What was notable were links found in 2 subgroups; a link between induced abortion and PR- breast cancer and the opposite association between miscarriage before the age of 20 and breast cancer incidence.
Among studies in which induced abortions were analyzed separately from miscarriages, no association was found in around half, and in the other half a greater breast cancer risk was detected. No link was established for miscarriage in most studies, including both group and case-control studies.
It is worth remembering that when discussing personal matters such as induced abortion underreporting is likely. Thus, general underreporting of induced abortion in a cohort study would most probably result in underestimation of the true relationship. Amongst NHSII participants, 66% were still pre-menopause at the end of follow-up period and so had not used post-menopause hormones, so their reproductive history may still have been incomplete.
Breast cancer cases were almost wholly pre-menopause; therefore, such findings may not extend to postmenopausal women who have used hormones. Despite the fact that such data does not match any significant overall relationship between induced abortion and breast cancer, a simple association cannot be ignored in subgroups defined by known breast cancer risk factors, timing of abortion, or parity.
Among this majorly pre-menopause population, neither induced nor spontaneous abortion was related to the occurrence of breast cancer. The data from the NHSII study supports earlier evidence that there is a lack of an important overall association between induced or spontaneous abortions and breast cancer risk even after Hazard Ratios (HRs) were altered for menopause status (pre-menopause or post-menopause), age at menopause, and post-menopause hormones use.