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Contraception - Background information
How do progestogen-only pills (POPs) prevent pregnancy?
- Progestogen-only pills (POPs), containing norgestrel, desogestrel, levonorgestrel, norethisterone, or etynodiol diacetate, work by thickening cervical mucus, delaying ovum transport, inhibiting ovulation, and providing an endometrium hostile to implantation [McCann and Potter, 1994; FFPRHC, 2005a].
- Five independent modes of action have been described [McCann and Potter, 1994]:
- Suppression of ovulation in about half of cycles — 97% in the case of desogestrel (Cerazette®) [Rice et al, 1999].
- Suppression of mid-cycle peaks of luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
- Production of 'hostile, blocked' mucus, resulting in poor sperm penetration.
- Reduction in the number and size of endometrial glands and inhibition of progesterone receptor synthesis in the endometrium, preventing implantation.
- Reduction in the activity of cilia in the Fallopian tube.
- POPs prevent conception (even when ovulation is not suppressed) through various combinations of these mechanisms, with great inter-individual and intra-individual variation [McCann and Potter, 1994].
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