## Mechanism Comparison: COCs vs POPs **Key Point:** Combined oral contraceptives (COCs) and progestin-only pills (POPs) differ fundamentally in their primary contraceptive mechanism. ### Combined Oral Contraceptives (COCs) 1. **Primary mechanism:** Suppress ovulation via inhibition of GnRH secretion and reduction of FSH and LH at the hypothalamic-pituitary level [cite:KD Tripathi 8e Ch 62] 2. The estrogen component suppresses FSH, preventing follicular development 3. The progestin component suppresses the LH surge, preventing ovulation 4. Secondary mechanisms include endometrial atrophy and cervical mucus thickening ### Progestin-Only Pills (POPs) 1. **Primary mechanism:** Do NOT reliably suppress ovulation 2. Work through secondary mechanisms: - Increased cervical mucus viscosity (blocks sperm transport) - Endometrial atrophy (prevents implantation) - Occasional suppression of LH surge (variable) 3. Less effective at preventing ovulation compared to COCs | Feature | COCs | POPs | |---------|------|------| | **Ovulation suppression** | Reliable (>99%) | Inconsistent (~20% of cycles) | | **Primary mechanism** | FSH/LH suppression | Cervical mucus + endometrial changes | | **Estrogen content** | Yes (ethinyl estradiol) | No | | **Progestin dose** | Lower | Higher | | **Timing requirement** | ±12 hours | ±3 hours (mini-pill) | **High-Yield:** The presence of estrogen in COCs enables reliable ovulation suppression; POPs lack this ovulation-suppressing capability and depend on local reproductive tract effects. **Clinical Pearl:** POPs are preferred in lactating women because estrogen can reduce milk production, whereas COCs are contraindicated in the first 6 months postpartum due to VTE risk.
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