How do contraceptives work?

For pregnancy to occur, sperm must swim from the vagina, through the cervix, uterus, and into one of the fallopian tubes to fertilize an egg. Contraceptives prevent this process in three ways: by blocking the sperm, disabling sperm before they reach the uterus, or suppressing ovulation.
Condoms are a simple form of barrier contraception that create a physical barrier to prevent sperm from reaching the vaginal area. They also help prevent certain sexually transmitted diseases.
Other barrier methods like the diaphragm, cervical cap, and sponge block, the cervix to prevent pregnancy. They can be used with spermicides which destroy and immobilize sperm.
Hormonal contraceptives, like the pill, patch, Depo shot, and vaginal ring, prevent pregnancy by releasing synthetic versions of progesterone and estrogen, that stop ovulation and make cervical mucus thick and sticky, making it difficult for sperm to swim.
Other contraceptives, like IUDs, use synthetic hormones or copper to prevent ovulation and disable sperm. How well a method is used affects its effectiveness. Hormonal methods can cause side effects like headaches, nausea, and high blood pressure, and require a prescription from a doctor.