## Ovulatory Surge Mechanism **Key Point:** The LH surge is the critical hormonal trigger for ovulation. It occurs 24–36 hours before the release of the oocyte from the mature follicle. ### Physiological Basis The LH surge is induced by a **positive feedback effect of estradiol** when estrogen levels exceed a critical threshold (typically >200 pg/mL for 24–48 hours) during the late follicular phase. ### Timeline of Events 1. Rising estradiol from maturing follicle → positive feedback on hypothalamus and anterior pituitary 2. **LH surge begins** (sharp, rapid rise) 3. **24–36 hours later** → ovulation occurs 4. Rupture of follicle wall and release of secondary oocyte ### Hormonal Changes During Surge | Hormone | Change | Timing | |---------|--------|--------| | LH | Sharp ↑↑ (10–20× baseline) | 24–36 hrs before ovulation | | FSH | Mild ↑ (2–3× baseline) | Concurrent with LH surge | | Estradiol | ↓ after surge begins | Negative feedback resumes | | Progesterone | Begins to rise | After LH surge | **High-Yield:** The LH surge is the **most reliable predictor of ovulation** and is used clinically in ovulation prediction kits (urine LH detection). **Clinical Pearl:** Absence of LH surge indicates anovulation, seen in polycystic ovary syndrome (PCOS), hypothyroidism, or hyperprolactinemia. **Mnemonic:** **"LH Launches Ovulation"** — LH surge is the launch sequence for egg release. 
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