The pill, a combined oral contraceptive consisting of estrogen and progesterone, has an average failure rate of 3-8%.
Keep in mind that all contraceptives have failed at some point. Even if you use your method perfectly, there are no guarantees -- it still might fail. If your method of contraception has an average failure rate of 18%, over five years your likelihood of pregnancy is greater than fifty percent. During those five years, figure 63 out of 100 women using a diaphragm will have gotten pregnant at least once. 20% of young women who become sexually active become pregnant within the first month of sexual activity. 50% become pregnant within the first six months. The average woman using reversible contraception can expect two unintended pregnancies in her lifetime, or more if she does not always use her method. Even a low annual risk of contraceptive failure implies a high risk of becoming pregnant during a lifetime of use.