Mechanism of Action of Contraceptive Pills
The primary mechanism of action of contraceptive pills is by inhibiting ovulation (option 1). Combined oral contraceptives (COCs) prevent ovulation and pregnancy by inhibiting gonadotropin-releasing hormone and, subsequently, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are essential for follicular maturation and ovulation 1.
Combined Oral Contraceptives (COCs)
Primary Mechanism: Ovulation Inhibition
- COCs suppress the mid-cycle surge of LH and FSH, preventing ovulation from occurring 2.
- Seven consecutive days of combined hormonal contraceptive use is necessary to reliably prevent ovulation 1.
- When COCs are missed during the hormone-free interval or early in the cycle, the risk of ovulation increases because follicular development can resume 1.
Secondary Mechanisms
While ovulation inhibition is the primary mechanism, COCs also work through additional pathways:
- Cervical mucus thickening: COCs cause the cervical mucus to become thicker and more viscous, which impairs sperm penetration and motility 3, 4.
- Endometrial changes: The hormones decidualize the endometrial bed and suppress mitotic activity, making it less receptive to implantation 2, 4.
Progestin-Only Pills (POPs)
Traditional progestin-only pills work differently than COCs—they primarily thicken cervical mucus rather than consistently inhibiting ovulation 1, 5.
Key Differences from COCs
- POPs do not consistently inhibit ovulation, which contributes to their lower effectiveness compared to combined methods 5.
- The primary mechanism is creating a barrier to sperm entry through cervical mucus thickening 1, 6, 5.
- Because POPs rely more on cervical mucus effects than ovulation suppression, they require very stringent adherence to timing (must be taken within 3 hours of the same time daily) 1, 5.
Addressing the Other Options
Option 2: Increasing Fallopian Tube Motility
Option 3: Decreasing Sperm Motility
- While cervical mucus thickening does impair sperm penetration and movement, this is a secondary mechanism, not the primary one for COCs 3, 4.
- This mechanism is more relevant for POPs than for combined pills 1, 5.
Option 4: Blocking Sperm Entry into Fallopian Tube
- Cervical mucus thickening does create a barrier to sperm, but this occurs at the cervical level, not specifically at the fallopian tube entrance 3, 4.
- For COCs, this is a secondary mechanism; ovulation inhibition remains primary 1.
Clinical Implications
The distinction between COC and POP mechanisms matters clinically:
- When COCs are missed for ≥48 hours, backup contraception is needed for 7 consecutive days because ovulation suppression is compromised 1.
- POPs require backup contraception for only 48 hours after a missed dose because their primary mechanism (cervical mucus) recovers more quickly 5.
- The multimodal mechanism explains why COCs remain highly effective even with occasional missed pills, as long as the hormone-free interval is not extended 1.