Anovulatory cycle
While the normal menstrual cycle in the human typically lasts 4 weeks (28 days, range 24–35 days) and consists of a follicular phase, ovulation, and a luteal phase, followed by either menstruation or pregnancy, the anovulatory cycle has cycle
lengths of varying degrees. In many circumstances, menstrual intervals are prolonged exceeding 35 days leading to oligomenorrhea (cycle >35 – 180 days interval), or even longer, amenorrhea. In other cases, menstruation may be fairly regular (eumenorrhea), or more frequent (intervals < 21 days), or there may be a loss of menstrual pattern (menorrhagia, dysfunctional uterine bleeding).
Human reproductive ecology
The regular menstrual cycle is associated with the hormonal regulation from hypothalamic, pituitary, and ovarian axis. Gonadotropin-releasing hormone (GnRH) secretes from the hypothalamus. Hypothalamic GnRH pulse influences the pulsatile secretion of Follicle stimulating hormone (FSH) and Luteinizing hormone (LH) from the pituitary gland. During the menstrual cycle, due to a decreased level of inhibin-A and steroid hormones, the level of FSH increases. Due to these hormonal changes, the corpus luteum gets destroyed. The elevated level of FSH helps to recruit a cohort of the FSH-sensitive antral follicles in that cycle. During this phase, elevated FSH level stimulates the production of estradiol, Inhibin A and B. Following that, due to the negative feedback mechanism the level of estradiol and Inhibin-B increases and FSH level declines and it helps to select the dominant follicle. During the menopausal transition, FSH level elevates at the early follicular phase and due to the increased FSH level, the number of FSH-sensitive follicles decreases. These series of events lead to irregular menstrual cycle and the cycle length starts to become shorter. FSH, Inhibin-B, and Anti-Mullerian hormone (AMH) are used as the biomarkers for the ovarian aging.