## Primary Mechanism of Combined Oral Contraceptives **Key Point:** Combined oral contraceptives (COCs) contain both estrogen and progestin, which work synergistically to suppress the hypothalamic-pituitary-ovarian (HPO) axis. ### Mechanism of Action 1. **Suppression of GnRH**: Estrogen and progestin inhibit GnRH secretion from the hypothalamus 2. **FSH and LH Suppression**: Decreased GnRH leads to reduced FSH and LH from the anterior pituitary 3. **Anovulation**: Without the LH surge, ovulation is prevented — this is the PRIMARY contraceptive mechanism 4. **Secondary mechanisms**: Increased cervical mucus viscosity and endometrial atrophy provide additional contraceptive effects **High-Yield:** The estrogen component is primarily responsible for suppressing FSH (follicle development), while the progestin component suppresses LH (ovulation trigger). Together, they achieve >99% efficacy when used perfectly. **Clinical Pearl:** Even if ovulation were to occur (rare breakthrough ovulation), the secondary mechanisms (altered cervical mucus and endometrial atrophy) provide backup contraceptive protection. ### Why Ovulation Prevention is Primary | Mechanism | Efficacy | Reversibility | | --- | --- | --- | | Ovulation suppression | ~99% | Immediate upon discontinuation | | Cervical mucus changes | ~60% (if used alone) | Reversible within days | | Endometrial changes | ~50% (if used alone) | Reversible within cycles | [cite:KD Tripathi 8e Ch 62]
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