Human Ovarian Follicular Dynamics and Emergency Contraception

Inclusion Criteria: 1. Female volunteers of childbearing potential; 2. Are first time users of oral contraception (OC) or have discontinued OC at least 1 month prior to study entry; 3. Age between 18 and 40 years old; 4. Normal body mass index (18-38); 5. Has signed informed consent form; and 6. Is in good health as confirmed by medical history, physical examination. Exclusion Criteria: 1. A positive pregnancy test will automatically exclude the volunteer from participation in this study. 2. Any contraindication for oral contraception use; 3. Irregular menstrual cycles; 4. Ultrasonographic evidence of ovarian dysfunction, such as polycystic ovary syndrome (PCOS); 5. Pregnancy (suspected or diagnosed) or lactation; 6. History or suspicion of drug or alcohol abuse; 7. Participation in an investigational drug trial within the 30 days prior to selection; 8. Exhibits a disorder that is a contraindication to steroid hormonal therapy, including, for example, the following conditions: * history of, or actual, thrombophlebitis or thromboembolic disorders. * history of, or actual, cerebrovascular disorders. * history of, or actual, myocardial infarction or coronary artery disease. * acute liver disease. * history of, or actual, benign or malignant liver tumors. * history of, or suspected, carcinoma of the breast. * known, or suspected, estrogen-dependent neoplasia. * undiagnosed abnormal vaginal bleeding. * any ocular lesion arising from ophthalmic vascular disease, such as partial or complete loss of vision or defect in visual field.