How does abortion affect a women's risk for breast cancer?

Updated: Dec 26, 2019
  • Author: Graham A Colditz, MD, DrPH; Chief Editor: Chandandeep Nagi, MD  more...
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Controversy remains regarding whether there is an association between induced abortion and a risk of breast cancer. [25]

Close to one-quarter of all clinically identified pregnancies in the United States end as induced abortions. [26] Breast cells may be most vulnerable to mutation when breast tissue consists of rapidly growing and undifferentiated cells, such as during adolescence and pregnancy. In early pregnancy, the number of undifferentiated cells increases as rapid growth of the breast epithelium is taking place. If the pregnancy continues to term, these cells differentiate by the third trimester, decreasing the number of cells susceptible to malignancy. The interruption of the differentiation of breast cells that results from spontaneous and induced abortions has been hypothesized to increase a woman’s risk of developing breast cancer.

By far the strongest study to date on the association between breast cancer and abortion was a population-based cohort study made up of 1.5 million Danish women born April 1, 1935, through March 31, 1978. [27]  Of these women, 280,965 (18.4%) had had one or more induced abortions. After adjusting for potential confounders of age, parity, age at delivery of first child, and calendar period, the risk of breast cancer for women with a history of induced abortion was not different from women who had not had an induced abortion (relative risk [RR] = 1.0; 95% CI, 0.94-1.06). The number of induced abortions in a woman’s history also had no significant relation to risk of breast cancer.

A statistically significant increase in risk was found among the very small number of women with a history of second-trimester abortion. Results from this population-based prospective cohort provide strong evidence against an increase in risk of breast cancer among women with a history of induced abortion during the first trimester. Comparable results have been observed in a large cohort of women in China [28] and the California Teachers Study. [29] Taken as a whole and accounting for the limitations of the case-control study design, the available evidence does not support any important relation between induced abortion and risk of breast cancer.

However, a 2018 meta-analysis of 25 studies in the English language literature up to December 10, 2016, indicates there is no association between induced abortion and breast cancer risk. [25]  A subgroup analysis suggested that induced abortion may raise the risk of breast cancer in parous women but not in nulliparous women. [25]

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