Hormonal contraceptives such as the combined pill contain oestrogen and progesterone, or progesterone alone. Taking them changes the menstrual cycle so that a woman is much less likely to become pregnant.
The oestrogen in the pill inhibits the release of FSH from the pituitary gland. Without FSH, no egg matures in the ovary, so ovulation does not happen and there is no egg for sperm to fertilise. Progesterone makes the mucus in the cervix thick, which stops sperm from getting through the cervix into the uterus to reach an egg.
The hormones are given in different ways: a pill taken every day, a skin patch, an implant under the skin, or an injection. In the combined pill, the hormones are usually taken for 21 days, then there is a short break, during which the uterus lining breaks down and a period happens.