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Plasma levels of LH, FSH, estradiol, and progesterone during the human ovarian cycle. Cycle phases, labeled with respect to the LH peak, are follicular (F), periovulatory (PO), and luteal (L). The follicular phase begins with menses, and the LH peak and ovulation occur ∼14 d later. Longer menstrual cycles are usually caused by prolonged follicular phases. Values are smoothed averages based on several sources (97, 617, 638, 735).
