The birth control pill is one of many methods of preventing pregnancy. It works by providing hormones to a woman on a continuous basis. Some contain one hormone, progestin, and others contain progestin and estrogen. The hormones in the pill prevent ovulation, or the release of an egg from the ovaries so a woman can't get pregnant. Oral contraceptives can alter a woman's menstrual period, making her more regular or she may miss a period occasionally.
New Birth Control
The American Congress of Obstetricians and Gynecologists points out that missing a period is a common side effect of birth control pills. This is due to the body adjusting to the new hormone levels. As a precaution, the woman should take a pregnancy test to be sure she isn't pregnant. Missing two periods in a row is not as common and should be reported to a medical care provider.
Constant Hormones
The Mayo Clinic points out that some forms of birth control are meant to be taken continuously to prevent a period for several cycles. When the woman stops taking active pills that contain hormones, she will have a period. Progestin-only versions of the pill, often called the mini-pill, are taken once every day. Other pills have one week of inactive, or placebo, pills that do not contain any active ingredients. The decrease in hormones during that week triggers a period. With the mini-pill, the hormone levels remain constant. Some women will still have a period with this type of birth control, but others may not or the cycle may be irregular, according to KidsHealth.org.
Pregnancy
Birth control pills lose their effectiveness in preventing pregnancy if they are not taken exactly as directed. This means taking one pill at the exact same time each day until the week of inactive pills, if applicable. Missing one pill can increase the risk of pregnancy. Missing two pills requires that a woman use a back-up birth control method, such as a condom, in most cases. A woman who suspects she may be pregnant should take a pregnancy test and consult a health-care provider about stopping the pill. Taking birth control while pregnant can have adverse effects on the pregnancy but does not increase the risk of birth defects, according to the American Congress of Obstetricians and Gynecologists.
