Oral contraceptives, often called the pill, contain the hormones oestrogen and progesterone. A woman takes one regularly, and this keeps the levels of these hormones high in her blood.
The high hormone levels inhibit the production of FSH by the pituitary gland. Without FSH, no eggs mature in the ovaries. With no mature egg, ovulation does not happen, so there is no egg for sperm to fertilise and pregnancy is prevented.
Progesterone also makes the mucus in the cervix thick, so sperm find it hard to pass through and reach an egg. Some hormonal methods, such as implants and injections, release progesterone only, and they work by the same ideas.