Mid-luteal Phase Synchronization of Ovarian Folliculogenesis in Women

Inclusion Criteria: 1. female volunteers of childbearing potential; 2. are first time users of OC or have discontinued OC at least 2 months prior to study entry; 3. age between 18 and 35 years old; 4. normal body mass index (18-30); 5. has signed 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. known hypersensitivity to Letrozole and co-administered medications; 4. irregular menstrual cycles; 5. ultrasonographic evidence of ovarian dysfunction, such as Polycystic Ovary Syndrome (PCOS); 6. history of pituitary tumor; 7. HIV, HBV, HCV infection; 8. vaginal infection; 9. abnormal ECG; 10. abnormal lab tests for blood profile, liver function and renal function; 11. uncontrolled diabetes and blood pressure; 12. pregnancy (suspected or diagnosed) or lactation; 13. history or suspicion of drug or alcohol abuse; 14. history of severe mental disorders; 15. participation in an investigational drug trial within the 30 days prior to selection; 16. 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.