Birth control pills contain a combination of hormones that is used to prevent ovulation (the release of an egg from an ovary). The pills contain a form of estrogen and a form of progesterone, which are both female hormones involved in conception. Birth control pills also have other effects that inhibit pregnancy. They cause the cervical mucous to thicken, which makes it harder for sperm to move toward the uterus, and they prevent the attachment of an egg to the uterus.