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Education

The Menstrual Cycle

Learn more about what happens during the menstrual cycle

mLpenguin

• 1 min read
The Menstrual Cycle
Photo by Jeremy Perkins / Unsplash

Cycle Overview

  • Average cycle length: ~28 days (range: 21–35)
  • Divided into:
    • Menstrual Phase (Days 1-5)
    • Follicular Phase (Days ~6-14)
    • Ovulation (~Day 14)
    • Luteal Phase (Days 15-28)

Hormones

  • GnRH (from hypothalamus) → FSH & LH (from anterior pituitary) → Estrogen & Progesterone (from ovaries)

Menstrual Phase

  • Days 1-5
  • Drop in estrogen & progesterone (from corpus luteum degeneration)
  • Shedding of the endometrium
    • Spiral arteries constrict, leading to ischemia and tissue sloughing
    • Blood, necrotic debris, and glandular tissue are expelled

Follicular Phase (Proliferative Phase)

  • Days ~6-14
  • FSH ↑ → stimulates follicle development
  • The dominant follicle produces increasing estrogen
  • Estrogen:
    • Promotes endometrial proliferation
      • Regrowth and thickening of the endometrial lining
      • Glands become straight and narrow
      • Stroma becomes denser and more cellular
      • Spiral arteries begin to elongate but remain uncoiled
    • Provides negative feedback on FSH/LH (early)
    • Provides positive feedback on LH (late phase)

Ovulation

  • Occurs ~Day 14
  • Triggered by LH surge due to high estrogen levels
  • Oocyte is released from the Graafian follicle
  • Basal body temperature ↑ due to progesterone

Luteal Phase (Secretory Phase)

  • Days 15-28
  • Corpus luteum forms → secretes progesterone (+ some estrogen)
  • Progesterone:
    • Stabilizes and maintains the endometrium
      • Glands become tortuous, coiled, and begin secreting glycogen-rich fluid
      • Stroma becomes edematous, preparing for implantation
      • Spiral arteries continue to develop and coil further
    • Inhibits GnRH, FSH, and LH (negative feedback)
  • If no fertilization:
    • Corpus luteum degenerates → ↓ estrogen & progesterone → Endometrial shedding → menstruation
  • If Fertilization Occurs
    • hCG from the trophoblast maintains the corpus luteum until the placenta takes over
    • Progesterone remains high → maintains the endometrium
    • No menstruation

Estrogen Evolves the endometrium

Progesterone Protects the endometrium