The failure rate of combined oral contraceptive pills is commonly reported as 0.3% with perfect use and approximately 9% with typical use. The mechanism by which COCPs provide their PRIMARY contraceptive effect is:

• A Inhibition of gonadotrophin surge preventing ovulation ✓
• B Thickening of cervical mucus preventing sperm penetration
• C Endometrial atrophy preventing implantation
• D Alteration of tubal motility preventing fertilisation

Explanation

The primary mechanism of combined oral contraceptive pills is suppression of the mid-cycle LH surge (and FSH) by the combined oestrogen-progestogen effect on the hypothalamo-pituitary axis, thereby preventing ovulation. Secondary mechanisms include cervical mucus thickening (progestogen effect) and endometrial changes. When ovulation is reliably prevented, the other mechanisms serve as backup; the high typical-use failure relates to inconsistent pill-taking, not mechanism failure.

Reference: Shaw's Textbook of Gynaecology, 17th ed.

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Written and medically reviewed by the StethoPrep medical team.