The Menstrual Cycle: Phases and Hormonal Regulation
The menstrual cycle is a coordinated 21-45 day process driven by pulsatile GnRH from the hypothalamus, which orchestrates pituitary gonadotropin release (LH and FSH) to regulate ovarian follicular development, ovulation, and endometrial preparation for implantation. 1
Hypothalamic-Pituitary Control
The hypothalamus functions as the cycle's pacemaker, releasing GnRH in pulses every 1-1.5 hours during the follicular phase and every 2-4 hours during the luteal phase 1. This pulsatile pattern is critical—it translates into coordinated LH and FSH secretion from the pituitary gland, which the ovary can respond to appropriately.
The tempo of GnRH pulses directly determines the pattern of gonadotropin release, making this neuroendocrine rhythm fundamental to normal cycle function 1.
The Three Phases
Follicular Phase
- FSH rises at the luteal-follicular transition, stimulating a cohort of follicles to grow and produce inhibin B in the early follicular phase 2
- By mid-follicular phase, a single dominant follicle is selected through mechanisms involving gonadotropin responsiveness, IGF binding protein regulation, and increased vascularization 2
- The dominant follicle increasingly secretes estradiol and inhibin A for approximately one week before ovulation 2
- Two-thirds of women show two follicle waves per cycle, while one-third show three waves—those with three waves have longer cycles and delayed estradiol rise 2
The follicular phase involves a "two-cell, two-gonadotropin" system: LH stimulates theca cells to produce androstenedione, while FSH stimulates granulosa cells to express aromatase (Cyp19), converting androstenedione to estradiol 1.
Ovulatory Phase
When estradiol from the dominant follicle reaches a critical threshold, it triggers positive feedback at the hypothalamus (likely through the kisspeptin system), causing increased GnRH secretion and the LH surge 1. This LH surge initiates the ovulation process, releasing the mature oocyte.
Luteal Phase
- After ovulation, the follicle transforms into the corpus luteum, which secretes progesterone, estradiol, and inhibin A in response to LH pulses 2
- The corpus luteum reaches peak size, secretion, and vascularization 6-7 days after ovulation 2
- The corpus luteum has a fixed lifespan—luteal regression occurs passively and is independent of the uterus, but can be prevented by hCG from the trophoblast starting 8 days after conception 2
- If pregnancy doesn't occur, declining steroid and protein hormone concentrations trigger the FSH rise that begins the next cycle 2
Endometrial Changes
The endometrium undergoes synchronized changes in response to ovarian hormones 3:
- Estradiol stimulates endometrial proliferation during the follicular phase
- Progesterone and estradiol together cause secretory differentiation during the luteal phase
- During the mid-luteal phase (when progesterone peaks), the secretory endometrium is optimally prepared for embryo implantation 1
- Without implantation, progesterone-responsive decidual cells initiate menstruation through prostaglandin release (PGE and PGF2α), vasoconstriction, and matrix metalloprotease secretion by leukocytes 2
Normal Cycle Parameters
Normal menstrual cycles last 21-45 days with 3-7 days of bleeding 4. While adolescents may initially experience anovulatory cycles after menarche, the vast majority of cycles become fairly regular 4.
Red Flags Requiring Evaluation:
- Absence of menarche by age 15
- Absence of menses for three consecutive months
- Menses lasting 8 days or longer (with or without heavy bleeding) 4
Clinical Significance
The menstrual cycle should be treated as a "vital sign"—normal menses indicate overall health, and abnormalities warrant timely investigation 4. The cycle reflects integrated function across multiple systems: the hypothalamic-pituitary-adrenal axis, autonomic nervous system, immune signaling, and even gut microbiota 5.
The cycle also creates metabolic variability, with small-to-moderate differences in energy intake, resting energy expenditure, and substrate utilization across phases—particularly increased energy intake and lipid oxidation during the mid-luteal phase compared to early follicular and peri-ovulatory phases 5.