The proposed study is a clinical trial in premenopausal women aged 18-39 to evaluate the capacity of daily Ulipristal Acetate (UPA) to reduce breast epithelial cell proliferation (increase in number of cells in the breast) and to measure its effect compared to that found with a combined estrogen-progestin oral contraceptive (COC). UPA is an anti-progestin in use as daily medication up to 12 months for the treatment of abnormally heavy bleeding at menstruation due to uterine fibroids, and is currently in trials in the U.S. to evaluate its use as a daily contraceptive.