Four cohort studies (648,694 women) and 28 case-control studies (36,999 cases): of these, 25 studied both types of abortion, 4 studied spontaneous abortion only, and 3 studied induced abortion only.
Breast cancer risk did not appear to be associated with an increasing number of spontaneous or induced abortions. Results from two studies of spontaneous abortion were statistically significant but in opposite directions. Four studies found that the association between breast cancer risk and induced abortion was statistically significant. It was suggested that breast cancer risk was not associated with the other measures of exposure to abortion, and unlikely to differ by age or a family history of breast cancer.
The breast cancer risk associated with abortion occurring before the first full-term pregnancy was similar to that after the first full-term pregnancy.
Several small studies showed that the association between the breast cancer risk and abortion appeared to be greater for nulliparous women than for parous women.