The development of breast cancer in women is related to the hormone levels in the body. Breast cell growth and division are influenced by estrogen, progesterone and lactogen. The levels of these hormones fluctuate most during pregnancy. Because miscarriage (including spontaneous abortion, induced abortion and stillbirth) is thought to interrupt the normal cycle of hormonal changes during pregnancy, the relationship between miscarriage and breast cancer has been studied by many authors. Many previous retrospective studies had concluded that miscarriage increases the incidence of breast cancer. However, the latest study, which summarizes the observational bias of previous studies and conducts large-scale prospective studies, suggests that neither spontaneous nor induced abortion increases the risk of breast cancer. The results of this study can be found in a recent report by the American Cancer Society.