Birth control pills work by inhibiting the ovulation process, and in a way hinders the movement of sperm by thickening the cervical mucus, thus, preventing the egg and sperm from meeting.
The use of oral contraceptives is up to 99% effective when used perfectly. Perfect use of contraception here means that you use birth control at the same time every day, never missing a dose. But if you take the pill at different times each day or accidentally skip a day, it’s only 91% effective at preventing pregnancy.
Although the percentage of effectiveness of birth control pills is very high, you can still get pregnant. Contraception failure is usually the result of missing a pill or more in the same course. If your body doesn’t get a constant supply of hormones, you may start to ovulate. Then, if you have sex without using a backup birth control, your chances of getting pregnant will increase.