Prevention of Ovulation
Ovulation is the part of the monthly menstrual cycle in which a woman releases an egg, waiting to be fertilized by sperm to create an embryo. Birth control pills, or oral contraceptives, contain hormones that keep ovulation from happening. Birth control pills typically contain a combination of estrogen and progesterone, which inhibit the release of an egg and prevent pregnancy.
Thickening of Cervical Mucus
The hormones in birth control pills have another effect on the reproductive system--they cause the mucus that covers the cervix to thicken. When cervical mucus gets thick, sperm can't reach an egg in the event that ovulation has occurred. If the egg can't be fertilized, pregnancy doesn't occur.
Changes in Uterine Lining
Estrogen and progesterone perform yet another function to prevent pregnancy. Birth control pills cause the tissues that line the uterus to become thinner than usual. When the uterine lining is abnormally thin, a fertilized egg can't attach to the lining, and pregnancy can't occur.
