Most combination birth control pills contain three weeks of active pills (those that contain hormones) and one week of inactive placebo pills (those that do not contain hormones). The bleeding of the period occurs when the hormones are no longer taken during the week that the sugar or placebo pills are taken. A woman can increase the length of time between periods by taking active pills for more weeks. Some drug companies make pill packs that contain up to 3 months of continuous active pills. Women on these pills only have four periods a year, which can be convenient during such times as final exams, sports activities, or social events.
You can find more information from the American Society for Reproductive Medicine on this topic here:
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