The failure rate is less than 0.2 percent a year.
Oral contraceptives contain two synthetically produced female hormones, estrogen and progesterone.
They cause the suppression of the pituitary gland that would normally stimulate the ovary to release an egg cell. Without the presence of an egg, conception can not take place. Oral contraceptives also act on the cervix, the lining of the uterus, and on the fallopian tubes, rendering them inhospitable not only to an egg or embryo, but to sperm as well. Pills are sold in packs geared to the normal 28-day menstrual cycle. In a typical pack there will be 21 active birth control pills and 7 placebos, vitamin and iron tablets.