Female reproductive cycle phases

Follicular Phase - Prepping the Palace

• Initiation: Pulsatile GnRH from the hypothalamus stimulates the anterior pituitary to release FSH & LH.
• Follicle Growth: FSH stimulates a cohort of primordial follicles to develop. One becomes the dominant Graafian follicle.
• Estrogen Production: LH stimulates theca cells to produce androgens; FSH stimulates granulosa cells to convert these to estrogen.
• Endometrial Effect: Rising estrogen levels induce the proliferative endometrium.

⭐ In the late follicular phase, sustained high estrogen levels switch from negative to positive feedback on the pituitary, causing the LH surge that triggers ovulation.

Ovulation - The Great Escape

• Trigger: A massive Luteinizing Hormone (LH) surge, which is induced by a sustained peak in estrogen from the mature Graafian follicle.
• Mechanism: Positive feedback loop.
  • When estrogen levels are sustained >200 pg/mL for ~48 hours, the feedback on the hypothalamus (GnRH) and pituitary switches from negative to positive.
  • This results in a dramatic ↑ LH release.

• Event: Occurs ~10-12 hours after the LH peak, typically on day 14.
  • The primary oocyte completes meiosis I.
  • The follicle ruptures, releasing the secondary oocyte.
• Clinical: May be associated with mittelschmerz (mid-cycle pain).

⭐ The LH surge is the critical trigger that allows the primary oocyte (arrested in prophase I) to resume meiosis and become a secondary oocyte (arrested in metaphase II).

Luteal Phase - The Waiting Game

• Duration: Fixed at 14 days, post-ovulation.
• Key Player: The corpus luteum (CL), formed from the ruptured follicle.
• Hormone: The CL primarily secretes progesterone.
  • 📌 Progesterone = Promotes Gestation.
  • Transforms the endometrium into a thick, vascular secretory endometrium, ideal for implantation.
• Two Fates:
  • No Pregnancy: Without hCG, the CL degenerates into a scar (corpus albicans). ↓Progesterone triggers menstruation.
  • Pregnancy: Embryonic hCG "rescues" the CL, maintaining progesterone until the placenta develops.

⭐ The luteal phase's fixed 14-day length is the most constant part of the menstrual cycle. Variability in total cycle length usually comes from the follicular phase.

Hormonal Axis - The Conductors

• The Hypothalamic-Pituitary-Ovarian (HPO) axis orchestrates the cycle via a hormonal cascade.
• Hypothalamus: Releases Gonadotropin-Releasing Hormone (GnRH) in pulses.
• Anterior Pituitary: GnRH stimulates Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) release.
• Ovaries: Respond to FSH & LH, producing estrogen, progesterone, and inhibin.
• Feedback: Generally, estrogen/progesterone provide negative feedback. Inhibin specifically suppresses FSH.

⭐ Sustained high estrogen from a mature follicle switches feedback from negative to positive, causing the LH surge that triggers ovulation approximately 24-36 hours later.

High‑Yield Points - ⚡ Biggest Takeaways

• The follicular phase is estrogen-dominant and driven by FSH, leading to endometrial proliferation.
• A massive LH surge is the direct and essential trigger for ovulation.
• The luteal phase is progesterone-dominant, maintained by the corpus luteum.
• Menstruation is triggered by the withdrawal of progesterone and estrogen.
• If pregnancy occurs, hCG "rescues" the corpus luteum to continue progesterone production.