Menstruation
Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days).
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.