## Mechanism of Action of Combined Oral Contraceptive Pills ### Primary Mechanisms **Key Point:** COCPs work through multiple overlapping mechanisms, with the primary effect being suppression of ovulation via hypothalamic-pituitary-ovarian (HPO) axis inhibition. ### Individual Component Actions | Component | Primary Action | Secondary Actions | |-----------|----------------|-------------------| | **Estrogen** | Suppresses FSH secretion | Inhibits follicular development; alters endometrial morphology | | **Progestin** | Suppresses LH surge (primary ovulation inhibitor) | Thickens cervical mucus; alters endometrium; reduces tubal motility | ### Correct Mechanisms (Options 0, 1, 2) 1. **Estrogen and FSH suppression** — Estrogen provides negative feedback on the anterior pituitary, suppressing FSH secretion and preventing follicular recruitment and development. ✓ 2. **Progestin and cervical mucus** — Progestin increases cervical mucus viscosity and creates a hostile environment for sperm penetration and survival. ✓ 3. **Progestin and LH suppression** — Progestin is the dominant ovulation inhibitor; it suppresses the LH surge by negative feedback on the pituitary and hypothalamus. ✓ ### Why Option 3 Is Incorrect **High-Yield:** Endometrial changes in COCP users involve **thinning and atrophy**, not thickening. The endometrium becomes thin, hypoplastic, and glandularly inactive — this is a **contraceptive advantage** (reduces implantation potential) but is NOT the primary mechanism of ovulation prevention. **Clinical Pearl:** The endometrial atrophy caused by COCPs is reversible and returns to normal after discontinuation. This is why breakthrough bleeding and amenorrhea can occur with prolonged COCP use. **Warning:** Do not confuse endometrial changes with the primary ovulation-suppressive mechanisms. Endometrial alterations are a **tertiary** contraceptive mechanism, not a primary one.
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