We hypothesize that administration of an aromatase inhibitor (AI) and hormonal contraceptives (HC) in the mid-luteal phase of the menstrual cycle will result in atresia of the follicles in the extant wave and cause synchronous re-emergence of a new follicular wave. We anticipate that this will provide us with information to facilitate the development of a new method for ovarian synchronization; a safer, more effective ovulation induction therapy; a new method for emergency contraception; and a greater understanding of human folliculogenesis.

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.