Combination pills usually work by preventing the ovaries from releasing eggs (ovulation).
They also thicken the cervical mucus, which keeps sperm from penetrating into the uterus and joining with an egg.
They are usually taken for 21 days with then a seven-day gap during which a withdrawal bleed (often, but incorrectly, referred to as a menstrual period) occurs.
The introduction of extended-cycle monophasic pills (i.e. Seasonale) has shown that the withdrawal bleeding intervals can be decreased.
Progestogen-only pills (POPs) use a progestogen alone with doses taken continuously and no or a short gap between packs taken.