Sure, you bleed during the week that you take the sugar pills, but technically that’s “monthly withdrawal bleeding.” It’s slightly different than a regular period.
Normally, you ovulate in the middle of your menstrual cycle. If the egg your ovaries release isn’t fertilized, your hormone levels drop, causing you to shed the lining inside your uterus, and you get your period.
Birth control pills, though, prevent ovulation. With most types, you take hormones for 3 weeks followed by 1 week of pills without them. Though they keep your body from releasing an egg, they don’t prevent it from building up the lining of your uterus all month. The period-like bleeding during that fourth week is your body’s reaction to the lack of hormones from the last week of the pill.