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How Do Oral Contraceptives Work?

Oral contraceptives contain hormones that act in a few different ways to prevent pregnancy. Most are classified as combined oral contraceptives and contain two different types of hormones: an estrogen and a progestin. The hormones in combined oral contraceptives prevent pregnancy primarily by stopping ovulation (the maturation and release of eggs from the ovaries). However, they also work to prevent pregnancy in two other, less important ways. Oral contraceptives change the cervical mucus (the fluid of the cervix, which is the lower, narrow part of the uterus that is connected to the vagina), making it more difficult for sperm to enter the uterus. They also alter the lining of the uterus (the endometrium), making it less receptive to an embryo.
Some oral contraceptives contain just a progestin and no estrogen. These are known as progestin-only contraceptives. These medications rely more heavily on changes in the cervical mucus and uterine lining, since they are less effective at preventing ovulation (only about half of women who take them stop ovulating). In comparison, combined oral contraceptives stop ovulation in almost all women (when taken correctly).

Different Kinds of Oral Contraceptives

There are several different types of oral contraceptives, and they can be categorized in a few different ways. One basic way to categorize them is by hormone content (combined versus progestin-only).
Oral contraceptives are also divided into types by the different "phases" in each pack (monophasic, biphasic, triphasic, or four-phasic). Some newer drugs are classified as extended-cycle oral contraceptives, which reduce or eliminate the number of menstrual periods per year. Sometimes, oral contraceptives are categorized by the strength of hormones (low-dose versus high-dose).
(Click Types of Birth Control Pills for more information.)
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Last reviewed by: Kristi Monson, PharmD;
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