Two of these additional ways happen after ovulation occurs but before the egg is fertilized:
- Changes occur in the cervical mucus
- Decreased ability of fallopian tubes to move the egg toward the sperm
- Estrogen generally initiates the thickening of the lining of the uterus in the first part of a female's menstrual cycle
- Progesterone then triggers the lining to mature
- Since either one or both of these hormones are being continuously supplied by hormonal birth control, the lining does not have the opportunity to thicken enough to nurture a fertilized egg
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