The Menstrual Cycle Explained
The menstrual cycle is a coordinated hormonal process lasting approximately 21-45 days (average 28 days) that prepares the female reproductive system for pregnancy through three distinct phases: follicular, ovulatory, and luteal 1, 2.
Hormonal Control System
The cycle operates through a precisely timed feedback loop between the hypothalamus, pituitary gland, and ovaries. The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses every 1-1.5 hours during the follicular phase and every 2-4 hours during the luteal phase. This pulsatile release stimulates the pituitary to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which then act on the ovaries 2.
The Three Phases
Follicular Phase (Days 1-14)
- Begins with menstruation (days 1-5): shedding of the endometrial lining when pregnancy doesn't occur
- FSH rises at the cycle's start, stimulating a cohort of ovarian follicles to grow
- One dominant follicle is selected around mid-follicular phase
- This dominant follicle increasingly secretes estradiol and inhibin, which thickens the endometrial lining
- Estradiol levels rise progressively as the follicle matures 2, 3
Ovulatory Phase (Day 14, approximately)
- High estradiol from the mature follicle triggers positive feedback, causing a surge in LH from the pituitary
- The LH surge initiates ovulation—the release of a mature egg from the dominant follicle
- This typically occurs around day 14 in a 28-day cycle, but timing varies with cycle length 2
Luteal Phase (Days 15-28)
- After ovulation, the ruptured follicle transforms into the corpus luteum
- The corpus luteum secretes progesterone, estradiol, and inhibin A in response to LH pulses
- Progesterone prepares the endometrium for embryo implantation, creating a secretory lining
- Peak corpus luteum function occurs 6-7 days after ovulation
- If pregnancy occurs, human chorionic gonadotropin (hCG) from the embryo maintains the corpus luteum
- Without pregnancy, the corpus luteum regresses after about 14 days, hormone levels drop, and menstruation begins 2, 3
Normal Cycle Characteristics
Normal menstrual bleeding lasts 3-7 days with cycle lengths of 21-45 days in adolescents and 21-35 days in adults 1. Two-thirds of women show two follicle waves per cycle, while one-third show three waves—the latter group typically has longer cycles 3.
Clinical Significance as a Vital Sign
The menstrual cycle should be treated as a vital sign from menarche through menopause 4, 1. Abnormal patterns warrant evaluation:
- Absence of menarche by age 15
- Absence of menses for three consecutive months
- Bleeding lasting more than 8 days
- Cycles shorter than 21 days or longer than 45 days (in adolescents) 1
Key Cellular Mechanisms
Within the ovarian follicle, LH stimulates theca cells to produce androstenedione, while FSH stimulates granulosa cells to produce aromatase, which converts androstenedione to estradiol. This two-cell, two-gonadotropin system is essential for normal ovarian function 2.
Endometrial Changes
The endometrium responds to hormonal fluctuations throughout the cycle:
- Proliferative phase: Estradiol stimulates endometrial growth
- Secretory phase: Progesterone causes differentiation and preparation for implantation
- Menstrual phase: Without pregnancy, progesterone withdrawal triggers prostaglandin release, vasoconstriction, and matrix metalloprotease secretion, leading to endometrial shedding 3
Common Variations
Adolescents may experience anovulatory cycles initially after menarche, but the majority of cycles become regular within 1-2 years. Cycle length variability of 26-35 days is normal in adult women 1, 3. The fertile window extends from 5 days before ovulation through the day of ovulation itself 3.