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In: Diczfalusy, E., ed. Regulation of human fertility. (Proceedings of the WHO Symposium on Advances in Fertility Regulation, Moscow, USSR, November 16-19, 1976) Copenhagan, Denmark, Scriptor, 1977. p. 21-71This chapter reviews the hormonal changes which occur during the menstrual cycle. During the last days of the preceeding menstrual cycle, plasma levels of luteinizing hormone (LH) and follicle stimulating hormone (FSH) increase. Follicular phase is characterized by gradually increasing estrogens. A few days preceding the LH surge, some little understood changes in estradiol, LH, and 17-hydroxyprogesterone, on one hand, and ACTH, cortisol, and aldosterone, on the other, occur. Evidence indicates that the estradiol peak occurs first, followed by a simultaneous rise and fall in LH and 17-hydroxyprogesterone values. The peak period of LH is about 32-44 hours long, during which time a rise in progesterone levels takes place. Other pituitary and steroid hormones (human chorionic gonadotropin, ACTH, prolactin, testosterine, androstenedione, cortisol, and aldosterone) show elevated levels during the periovulatory period. Ovulation occurs 16-48 hours after LH peak. The period following LH surge is characterized by rapidly increasing levels of progesterone, 17-hydroxyprogesterone, and 20-alpha-dihydroprogesterone, accompanied by moderately increasing estrogen levels to form the typical luteal-phase hormonal pattern. A luteal increase occurs also in levels of several other hormones, ranging from renin activity to angiotension, or from pregninolone to aldosterone. The last part of the luteal phase is characterized by rapidly declining levels of peripheral hormones. The perimenstrual phase around onset of heavy bleeding is characterized by gradually decreasing levels of progesterone, 20-alpha-hydroprogesterone, estradiol, and testosterone, associated with an incipient rise in LH and FSH levels.