Ovulatory cycle (human)

The normal ovulatory cycle in women is a complex and important process which repeats throughout the childbearing years. The cycle centers on the release of an oocyte from the ovary, and it is this event that is a basic determinant of female fertility.

Hormonal events of the normal ovulatory cycle

(1) Rising follicle-stimulating hormone (FSH) levels that cause follicular recruitment.7 (2) Follicular development that causes an increase in estradiol levels.8, 9 (3) Increasing estradiol levels, secreted by maturing follicles, cause endometrium proliferation and an increase in the amount of mucus secreted by the cervical epithelium as well as change in the type of the mucus and the degree of the opening of the cervical os.10, 11, 12, 13, 14 (4) Follicular dominance.15 (5) Estrogen together with inhibin shuts off FSH while the dominant follicle continues to grow.16 (6) Estrogen secreted by the dominant follicle feeds back negatively on the hypothalamo- pituitary axis.17, 18 (7) The pituitary LH rises to LH peak and luteinization of the follicle is initiated. The ovum is released from the follicle (the ovum has a limited life span, 12 to 24 hours).18 (8) With the initiation of follicular luteinization, secretion of progesterone commences in the follicle. This initial rise in progesterone maintains the plateau of LH during the LH peak.17, 19 (9) This pattern of LH secretion aids the formation of a normal corpus luteum and an adequate luteal phase of the menstrual cycle.9, 17, 19 (10) The corpus luteum produces progesterone and estrogen.9, 20 (11) Progesterone changes the endometrium to the secretory type. It also affects the cervical mucus, converting it from estrogenic to progestational type, which is not suitable for sperm transport through the cervix. If fertilization has not occurred, the corpus luteum begins to regress after 6–7 days.13, 21, 22 (12) Estrogen and progesterone levels return to early follicular phase levels approximately 14 days after the initial formation of the corpus luteum.7, 23 (13) This drop in sex hormone levels releases the suppression of FSH and LH and a new cycle commences.7, 24 (14) Throughout the cycle the hypothalamus continues to produce gonadotropin-releasing hormone (GnRH) and secretes it continuously in a pulsatile fashion.25, 26