The normal menstrual cycle in women

Area of Science:

• Reproductive Endocrinology
• Gynecology
• Comparative Physiology

Background:

• The menstrual cycle is a complex hormonal process with significant variability in length and phases.
• Key hormones like FSH, LH, estradiol, progesterone, and inhibin regulate follicular growth, ovulation, and the corpus luteum.
• Endometrial changes, including proliferation and shedding, are crucial for potential pregnancy or menstruation.

Purpose of the Study:

• To provide a comprehensive overview of the physiological events during the human menstrual cycle.
• To highlight the hormonal regulation, follicular dynamics, and endometrial transformations.
• To explore similarities between human menstrual cycles and estrous cycles in other monovulatory species.

Main Methods:

• Review of existing literature on menstrual cycle physiology.
• Analysis of hormonal fluctuations and their impact on ovarian and endometrial function.
• Comparative analysis of menstrual and estrous cycles.

Main Results:

• Menstrual cycle length varies (26-35 days) with a 5-day menses and a fertile window around ovulation.
• Follicular development involves FSH stimulation, dominant follicle selection, and estradiol/inhibin secretion.
• The corpus luteum produces progesterone and estradiol; luteal regression is typically passive but can be prevented by hCG.
• Endometrial shedding is initiated by progesterone withdrawal and executed by prostaglandins and leukocytes.
• Two-thirds of women exhibit two follicle waves, while one-third show three waves per cycle, influencing cycle length.

Conclusions:

• The menstrual cycle is a precisely regulated process involving intricate hormonal feedback loops and distinct ovarian and endometrial phases.
• Variations in follicle wave patterns can influence cycle length and hormonal events.
• Comparative studies with estrous cycles offer valuable insights into reproductive physiology in monovulatory species.