The hormones in the pill, progestin (which mimics progesterone) and estrogen, decrease the release of GnRH, and therefore the release of FSH and LH. This restricts the follicles from growing, and by extension, an egg from growing and releasing from the ovary. Essentially, these synthetic hormones trick the ovary into thinking that it's already released an egg. The endometrium still builds in the uterus and is released, but this is known as a "withdrawal" period. It's the body's reaction to the withdrawal of the normal hormonal cycle. This is why a period while on the pill is usually lighter and shorter; the corpus luteum can't grow unless the ovary has released an egg. The progestin may also make it harder for sperm to enter the fallopian tube by making the vaginal mucus thicker.
You'll notice that the pill doesn't completely stop the release of GnRH or other hormones. This is an important distinction. While it's not common, some women who take the pill still ovulate and become pregnant. Most often, it's because they didn't take the pill at the same time each day or missed one or more pills. In general, the pill must be taken consistently and regularly to maintain the correct balance of hormones. For a small number of women, their own hormones are no match for the synthetic ones used in the pill and they still become pregnant. Among women who take it correctly, the pill failure rate is 0.3 percent [source: Contraceptive Technology].
All women are different, which is why the pill works to varying degrees of effectiveness. Some also experience side effects. We'll look at these next.