The researchers hypothesize that emergency contraception (EC) will initiate the changes associated with atresia or ovulatory failure at all stages of follicular development and that the image attributes associated with atresia will be similar regardless of the diameter of the dominant follicle when ovarian suppression is initiated. Second, changes associated with atresia may be observed, but ovulation of the dominant follicle is unimpeded.

This study is a single-center, randomized, open-label, double-controlled protocol to study the pattern of ovarian follicular growth and regression in two groups of women. The first group will use Plan B, which is a progesterone only OC containing 0.75 levonorgestrel. The second group of women will use the Yuzpe regimen, which uses 50 mg ethinyl estradiol/0.5 mg levonorgestrel pills at different stages of the menstrual follicular cycle.

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.