How do progestogen-only contraceptive pills work?
Progestogen-only pills prevent the mucus changes around the time of ovulation so that sperm cannot get through to the egg thus preventing fertilisation. The lining of the womb (endometrium) is kept thin so that, even if an egg is fertilised, it is unlikely to be able to implant and develop. The combined oral contraceptive pill (COP), with occasional exceptions, prevents egg release (ovulation). It is now recognised that the progestogen-only pill also interferes with ovulation although not as effectively as COPs.
