## Mechanism of Ovulation Prevention by Combined OCPs **Key Point:** Combined oral contraceptive pills (containing both estrogen and progestin) prevent ovulation primarily through suppression of gonadotropins (FSH and LH) at the pituitary level. ### Primary Mechanism: Gonadotropin Suppression The estrogen and progestin components of OCPs exert **negative feedback** directly on the anterior pituitary gland, suppressing the secretion of: - **Follicle-Stimulating Hormone (FSH)** — prevents follicular development - **Luteinizing Hormone (LH)** — prevents the LH surge necessary for ovulation Without the LH surge, ovulation cannot occur, making this the most critical contraceptive mechanism. **High-Yield:** The pituitary suppression is the PRIMARY mechanism. While GnRH inhibition occurs secondarily through negative feedback at the hypothalamus, the direct pituitary effect is more important and more consistently demonstrated. ### Secondary Mechanisms OCPs also employ backup contraceptive mechanisms: | Mechanism | Effect | Importance | | --- | --- | --- | | Cervical mucus thickening | Impedes sperm penetration | Secondary (progestin effect) | | Endometrial atrophy | Reduces implantation potential | Tertiary (progestin effect) | | Altered tubal motility | Reduces gamete transport | Minor | **Clinical Pearl:** Even if ovulation were to occur (rare breakthrough ovulation), the altered endometrium and cervical mucus would provide backup contraceptive protection, resulting in the high efficacy of OCPs (>99% with perfect use). **Mnemonic:** **STOP-OV** — Suppression of pituitary (FSH/LH) is the primary mechanism, Other mechanisms (cervical mucus, endometrium) are secondary.
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