The Menstrual Cycle & Hormonal Regulation
A Step 1-focused walkthrough of the menstrual cycle: the HPO axis and two-cell theory, the biphasic estrogen feedback switch that drives the LH surge and ovulation, and the fixed 14-day luteal phase — plus the hormone effects and clinical cues boards love to test.
One axis, two synchronized cycles
The menstrual cycle is best understood as two synchronized cycles driven by one hormonal axis. The ovarian cycle (follicular phase → ovulation → luteal phase) tracks the follicle; the uterine/endometrial cycle (menstrual → proliferative → secretory) tracks the lining that responds to it. Both are conducted by the hypothalamic–pituitary–ovarian (HPO) axis: pulsatile GnRH drives pituitary FSH and LH, which direct ovarian estradiol and progesterone output. GnRH must be pulsatile — the pulse frequency itself tunes the output (faster pulses favor LH, slower favor FSH).
One anchor unlocks the timing: the luteal phase is fixed at ~14 days (the lifespan of the corpus luteum), so all variation in cycle length comes from the follicular phase. Day 1 = first day of menses; ovulation ≈ day 14 in an idealized 28-day cycle.
Where estradiol comes from: the two-cell theory
Estradiol is made by a two-cell, two-gonadotropin collaboration:
- LH stimulates ovarian theca cells to convert cholesterol into androgens (androstenedione) — theca cells lack aromatase and cannot finish the job.
- These androgens diffuse to adjacent granulosa cells, where FSH upregulates aromatase, converting them into estradiol (granulosa cells lack the 17α-hydroxylase/17,20-lyase needed to make their own androgens — hence the partnership).
FSH also drives granulosa proliferation and induces LH receptors on the dominant follicle, priming it for ovulation. Inhibin (from granulosa cells and later the corpus luteum) selectively suppresses FSH, sharpening selection of a single dominant follicle.
- Luteal phase = fixed ~14 days; follicular phase is variable → sets cycle length
- GnRH must be pulsatile: fast pulses favor LH, slow pulses favor FSH; continuous GnRH agonist (e.g., leuprolide) downregulates receptors → suppresses FSH/LH after an initial flare
- Theca: LH → androgens; Granulosa: FSH → aromatase → estradiol (two-cell theory)
- Estrogen feedback is biphasic: low/moderate = negative; high & sustained (~200 pg/mL for ~2 days) = positive → LH surge
- LH surge triggers ovulation ~36 h after surge onset (~10–12 h after LH peak) and completes meiosis I
- Progesterone (from the corpus luteum): maintains secretory endometrium, raises basal body temp ~0.5°C, thickens cervical mucus
- Inhibin B (follicular) and inhibin A (luteal) selectively inhibit FSH
- No pregnancy → corpus luteum involutes → estrogen/progesterone fall → menses; pregnancy → hCG (LH-like) rescues the corpus luteum
- Peak progesterone ≈ day 21 (mid-luteal) — used clinically to confirm ovulation
Follicular vs. luteal phase
| Feature | Follicular phase | Luteal phase |
|---|---|---|
| Length | Variable | Fixed (~14 days) |
| Dominant hormone | Estradiol | Progesterone |
| Key ovarian structure | Growing/dominant follicle | Corpus luteum |
| Endometrium | Proliferative (straight glands) | Secretory (coiled glands, glycogen) |
| Gonadotropin role | FSH drives follicle growth | LH maintains corpus luteum |
| Estrogen feedback | Negative → flips to positive | Negative (with progesterone, inhibin A) |
| Basal body temp | Lower | Elevated ~0.5°C (progesterone) |
The feedback switch, ovulation, and meiosis
The pivotal concept is the estrogen feedback switch. Through most of the follicular phase, rising estradiol exerts negative feedback, restraining FSH so only the dominant follicle survives. Once estradiol is high and sustained, feedback flips to positive (it raises GnRH pulses and pituitary sensitivity to GnRH), producing the mid-cycle LH surge.
That surge does three things: it triggers ovulation (~36 h later), it drives the arrested primary oocyte to complete meiosis I (yielding a secondary oocyte arrested in metaphase II), and it luteinizes the ruptured follicle into the corpus luteum. If fertilization occurs, meiosis II completes; if not, the corpus luteum regresses after ~14 days into a corpus albicans, progesterone withdraws, and menses follows.
Meiotic arrest is a favorite trap. "An egg MET a sperm" — the secondary oocyte is arrested in METaphase II until it MEETs a sperm (fertilization completes meiosis II).
Remember the upstream block too: the primary oocyte sits in prophase I from fetal life until the LH surge each cycle. So:
- Prophase I → held until ovulation
- Metaphase II → held until fertilization
- ~0.5°C sustained rise in basal body temperature = the post-ovulatory progesterone rise (a retrospective sign of ovulation), not the LH surge itself.
- Mittelschmerz: unilateral mid-cycle pelvic pain from follicular rupture at ovulation.
- Spinnbarkeit / ferning: thin, stretchy, estrogen-dominant cervical mucus peaks just before ovulation; progesterone later makes it thick and scant.
- Next best step to confirm ovulation: a mid-luteal (day-21) serum progesterone — an elevated value confirms a corpus luteum formed. Home ovulation kits instead detect the urinary LH surge.
- Anovulation (e.g., PCOS) → unopposed estrogen → irregular/absent menses and endometrial hyperplasia risk.
Estrogen vs. progesterone effects
| Target | Estrogen | Progesterone |
|---|---|---|
| Endometrium | Proliferation (builds lining) | Secretory maturation (maintains lining) |
| Cervical mucus | Thin, watery, stretchy (ferning) | Thick, scant, impermeable |
| Basal body temp | No rise | Raises ~0.5°C |
| Feedback (typical) | Biphasic (neg → positive surge) | Negative |
| Myometrium | ↑ excitability, ↑ oxytocin receptors | ↓ excitability ("pro-gestation") |
| Main source | Granulosa/follicle → CL, placenta | Corpus luteum → placenta |
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