In the menstrual cycle, the LH surge that triggers ovulation typically occurs approximately 36 hours before follicular rupture. The primary trigger for the LH surge is:

• A Sustained high estradiol (>200 pg/mL for >50 hours) switching on positive feedback on GnRH/LH ✓
• B Sudden fall in estradiol at mid-cycle switching off negative feedback on GnRH
• C Rising progesterone from the dominant follicle stimulating GnRH pulse frequency
• D FSH-mediated upregulation of LH receptors causing auto-amplification

Explanation

At most times in the cycle, estradiol exerts negative feedback on hypothalamic GnRH and pituitary LH/FSH secretion. However, when estradiol levels are sustained above approximately 200 pg/mL for more than 50 hours (as occurs with the dominant follicle's peak secretion), this switches to positive feedback, triggering the massive GnRH pulse and pituitary LH surge. This is a classic example of a biological system with two modes: threshold-dependent sign reversal of feedback. The LH surge then activates resumption of meiosis and ovulation.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.