In general, birth-control pills work by

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Birth control pills work by preventing ovulation (the release of an egg from the ovary) or by making it difficult for the sperm to reach the egg. They can also affect the lining of the uterus, making it less receptive to a fertilized egg, which can prevent implantation. However, they do not cause an implanted embryo to separate from the uterine wall or create a toxic environment for gametes. They also do not abort pregnancies in the first week of development, as they prevent fertilization or implantation in the first place.

In General, Birth-Control Pills Work by