Anovulatory PCOS — Follicles Arrest, No Egg Released
Polycystic ovaries hold many small follicles that never mature.
- 6–13%: PCOS prevalence (reproductive-age women)
- 70–80%: Anovulatory infertility share (of PCOS infertility cases)
- 12+: Antral follicle count (small, arrested follicles)
- ~2–3:1: LH:FSH ratio (elevated and skewed)
Follicular arrest
Small follicles stall before reaching a dominant size.
Chronic anovulation blocks the LH surge needed to trigger ovulation.
Hormonal imbalance
Excess androgens and insulin resistance disrupt normal signaling.
Why induction is needed
Ovulation induction restarts the hypothalamic-pituitary-ovarian axis pharmacologically.
Letrozole Blocks Estrogen Synthesis At The Ovary
Letrozole halts aromatase, cutting estrogen production directly at the follicle.
- >97%: Aromatase inhibition (enzyme activity suppressed)
- ↓60–80%: Estrogen drop (circulating estradiol falls)
- ↑40–60%: FSH rise (reduced negative feedback)
- ~45 h: Half-life (short-acting, self-limiting)
Aromatase enzyme
Granulosa cells can no longer convert androgens into estrogen.
Blocking the ovary itself keeps brain estrogen receptors untouched.
Central feedback
Low estrogen removes the brake on hypothalamic GnRH pulses.
Monofollicular tendency
Local androgen buildup favors selecting a single follicle.
Clomiphene Blocks Estrogen Receptors In The Brain
Clomiphene occupies hypothalamic receptors, tricking the brain into low-estrogen mode.
- Weeks: Receptor occupancy (long receptor dwell time)
- ↑50–70%: FSH rise (centrally driven surge)
- Central: Antiestrogenic site (hypothalamus and pituitary)
- 5–7 d: Half-life (longer systemic exposure)
Selective estrogen receptor modulator
Clomiphene competitively blocks estrogen from binding its receptor.
The brain misreads normal estrogen as deficient, boosting FSH output.
Peripheral anti-estrogen effects
Blocked receptors also thin the endometrium and cervical mucus.
Multifollicular tendency
Weaker follicle selection lets several follicles grow together.
Rising FSH Recruits And Grows Ovarian Follicles
Circulating FSH binds granulosa cells, driving follicles toward maturity.
- ~2 d: FSH threshold window (recruitment sensitivity period)
- ~2 mm/d: Follicle growth rate (during stimulation)
- 18–24 mm: Mature follicle size (triggers ovulation readiness)
- US scan: Monitoring method (tracks follicle count/size)
FSH threshold
Only follicles above the FSH threshold keep growing.
Higher dose widens the threshold window, recruiting more follicles.
Granulosa proliferation
FSH drives granulosa cell division and estrogen output.
Dose-dependent recruitment
Standard and high doses raise follicle count and multiples risk.
Comparing Ovulation Rate And Multiple-Pregnancy Risk
An LH surge triggers release of one or more mature eggs.
- 61.7%: Letrozole ovulation rate (per treatment cycle)
- 48.3%: Clomiphene ovulation rate (per treatment cycle)
- 3.4%: Letrozole multiples rate (twin pregnancy risk)
- 7.4%: Clomiphene multiples rate (twin pregnancy risk)
LH surge
A sharp LH spike ruptures the dominant follicle wall.
Letrozole favors single-egg ovulation, lowering multiple-birth risk substantially.
Live birth outcomes
Letrozole shows higher live birth rates in PCOS trials.
Clinical guideline
Major guidelines now recommend letrozole as first-line therapy.