Study to Evaluate the Safety and Efficacy of Two Contraceptive Vaginal Rings Delivering a Daily Dose of 1500 or 2500 μg of CDB-2914

Inclusion Criteria: * Healthy women of reproductive age (21-40 years) * Not at risk for pregnancy based on one of the following * subject has undergone sterilization * subject is monogamous and her male partner has undergone sterilization * subject agrees to be abstinent for the entire duration of the study (Oregon site only) * subject does not have sex with men (Oregon site only) * Have regular menstrual cycles of 25-35 days duration * Have an intact uterus and both ovaries * Will be able to comply with the protocol * Capable of giving informed consent Exclusion Criteria: * Women participating in another clinical trial * Women not living in the catchment area of the clinic * Known hypersensitivity to progestins or antiprogestins * Known hypersensitivity to silicone rubber * Any chronic disease * All contraindications to oral contraceptive use, including * Thrombophlebitis or thromboembolic disorders * Past history of deep vein thrombophlebitis or thromboembolic disorders * Past or current cerebrovascular or coronary artery disease * Migraine with focal aura * Known or suspected carcinoma of the breast * Carcinoma of the endometrium or other known or suspected estrogen-dependent neoplasia * Undiagnosed abnormal genital bleeding * Cholestatic jaundice of pregnancy or jaundice with prior pill use * Hepatic adenomas or carcinomas * Known or suspected pregnancy * Desire to get pregnant during the study (through the use of reproductive technology for sterilized women or vasecto…