First principle: sustained estrogen flips feedback
For most of the cycle estrogen exerts negative feedback. But estradiol above a threshold, sustained for ~2 days, flips to positive feedback → a massive LH surge → ovulation ~32–36 hours after surge onset (~10–12 hours after the LH peak).
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The discriminator students miss
Detecting ovulation (matching test to purpose)
| Method | What it detects | Timing |
|---|---|---|
| Urinary LH kit | the surge | predicts ovulation ~24–36 h ahead |
| Mid‑luteal progesterone | that a corpus luteum formed | ~7 days pre‑menses |
| Serial ultrasound | follicular collapse | real time |
| Basal body temperature | post‑ovulatory progesterone thermogenesis | retrospective only |
Pitfalls
- Using BBT to plan intercourse — it confirms ovulation only after it has happened.
- Timing progesterone rigidly at "day 21" regardless of cycle length (Concept 13).
Pearls
- 🎯 Surge → ovulation ≈ 36 hours. LH kits exploit exactly this.
- 🩺 At ovulation the primary oocyte completes meiosis I → secondary oocyte (arrested in metaphase II until fertilisation).
Next → The follicle is now a gland — how long does it last and what sustains it? (Concept 10).