The man-made female hormones in the pills change a woman's natural hormone levels and prevent her ovaries from releasing an egg each month. The hormones also help prevent pregnancy in 2 other ways. They cause a thickening of the mucus on the cervix and they change the lining of the uterus. The thickened mucus on the cervix makes it hard for sperm to enter the uterus. The change in the lining of the uterus helps prevent a fertilized egg from attaching to the uterus.
