Figure 6. Changes in gonadotropins and ovarian steroids at midcycle, just prior to ovulation. The initiation of LH surge is at time 0. Abbreviations: E2, estrogen; P, progesterone (From Hoff JD, Quigley ME, Yen SCC. Hormonal dynamics at midcycle: A re-evaluation. J Clin Endocrinol Metab. 57:792, 1983.