Cell proliferation in the breast occurs throughout the menstrual cycle. The actions of hormones on the breast are rapid and an anti-progestin such as UPA, which will block the action of progesterone in the breast, would be predicted to quickly lower breast cell proliferation in premenopausal women. Effects of UPA on the uterus continue to be studied and are reassuring. COC use has not been found to lower breast cell proliferation and is not associated with any decrease in risk of breast cancer.