How does the combined pill work?
The combined pill works primarily by preventing the release of eggs from the ovaries (the release of eggs is called ovulation). Combined pills also change the cervical mucus and the endometrium. These changes make it difficult for sperm to move through and reach an egg. With ‘perfect use’ (taking the pill EVERY DAY at the same time) the failure rate is 0.3% and with ‘typical use’ (including inconsistent or incorrect use) the failure rate is 9%.47