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How do Contraceptives Work?

Contraceptives are methods of preventing or ending pregnancy, also referred to as “birth control,”and work in three ways:

  1. Prevent Fertilization. Fertilization is the union of a man’s sperm and woman’s egg. Some contraceptives create a physical barrier to block sperm from reaching an egg. Male and female condoms and the diaphragm are examples.

  2. Make the Uterus Hostile to Implantation.* Contraceptives can attempt to prevent a baby from implanting in the mother’s uterus after fertilization by creating a hostile environment. Intra-uterine Devices (IUDs) and chemical methods are in this category.

  3. Alter Body Chemistry.* Typically referred to as “hormonal” contraceptives, synthetic steroids like the pill mimic hormones to prevent pregnancy by changing a woman’s body chemistry, which has three impacts:

  • Prevent ovulation, the release of the egg each menstrual cycle.

  • Produce less and thicker mucus in the cervix so that sperm cannot easily enter the uterus.

  • Thin the lining of the uterus, making it more difficult for a fertilized egg to implant.


Chemical contraceptives include the oral birth control pill, Depo-Provera injection, patch, and implant. Chemical methods provide no protection against STDs and can actually increase your risk of getting an STD by making your reproductive tract more vulnerable to infection. [See Birth Control and STDs]



*Note: Methods in categories 2 and 3 can be ‘abortifacient,’ meaning that they end the life of a developing baby.

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