Birth control pills prevent pregnancy through several mechanisms, mainly by stopping ovulation. If no egg is released, there is nothing to be fertilized by the sperm, so the woman can't get pregnant. Most birth control pills contain synthetic forms of 2 female hormones, Estrogen and Progestin. These hormones stabilize a woman's natural hormone levels, and prevent Estrogen from peaking mid-cycle. Without the Estrogen bumps, the pituitary gland doesn't release other hormones that normally cause ovaries to release mature eggs.