When used correctly, birth control pills are about 99 percent effective when used alone to prevent pregnancy. However, the American Congress of Obstetricians and Gynecologists report that for typical users, this number is closer to 92 percent. This is because pills may either be forgotten, or not completely absorbed.
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A missed birth control pill means a missed dose of a set amount of hormones. If a woman misses her birth control pill dose (and especially if she misses more than one in a row), she may experience some light spotting. Spotting is the term for abnormal bleeding between periods. This spotting may occur even if she makes up the missed pill, according to the American Congress of Obstetricians and Gynecologists. Light bleeding between periods is not harmful, and is only temporary.
If a woman misses a dose of her birth control pills, she is often told to make up the missed pill as soon as possible. This depends on the type of birth control pill she is taking, as well as the time during the menstrual cycle that the pill was missed. Taking an extra dose may cause some minor nausea, as the amount of hormones ingested is often double on the make-up day. This nausea is perfectly normal, and is generally short-lived.
While the risk of becoming pregnant after missing a birth control pill depends largely on the timing within the menstrual cycle and the type of pill being taken, it is generally considered safe to miss one. Until recently, it was accepted that up to three birth control pills could be missed without increasing the risk of an unwanted pregnancy; however, a study in the Lancet reports that this guideline was not well founded and may be misleading. Currently, the American Congress of Obstetricians and Gynecologists suggests that women who miss two or more pills in a row use a back-up contraceptive, such as a condom or spermicide, to decrease their risk of becoming pregnant.
Nothing, for Certain Types of Pills
Birth control pills come in many different strengths and dose types. Some pills are taken for up to three months without a break, and will result in a woman having only four periods per year. Others are taken for three weeks, and skipped for seven days, during which time a woman's period will begin. Some types of birth control pill have 21 active pills, and seven placebo pills. If a woman misses one or more birth control pills during the seven placebo days, nothing will happen. This is because these placebo pills contain inactive ingredients only. A missed pill during this time does not need to be made up: it can be thrown away without consequences.