Irregular menstrual cycles are common in girls for several years after their first menstrual period. The cause of abnormal menstrual cycles during this time is not well-understood. The purpose of this study is to: 1) monitor girls during a menstrual cycle (with blood and urine sampling and serial pelvic ultrasounds) to identify those girls who do not ovulate (release of an egg from the ovary), and 2) determine whether cycles can be corrected by treating girls with a short course of low-dose estrogen and progesterone.

Inclusion Criteria:

• no more than 3 ½ years postmenarchal

Exclusion Criteria:

• Subjects currently on or previously treated with medications that may affect reproductive hormones (eg birth control pills).
• Subjects with severe acne or hirsutism
• Subjects who exercise excessively (running > 20 miles/week or its equivalent)
• Subjects with any of the following medical conditions: diabetes, hypertension, hyperlipidemia, valvular heart disease, lupus, rheumatoid arthritis, migraine headaches with aura, undiagnosed breast mass, inflammatory bowel disease, gallbladder disease, sickle cell disease, or thrombophilia.
• Current smoker
• History of deep venous thrombosis or pulmonary embolism in subject or first-degree relative