Why does missing birth control pills cause bleeding?

Answer 1

When taking Birth Control pills the bleeding you get each month is withdrawal bleeding and not a "regular" period that comes on its own. So when you miss a few pills the withdrawal from them starts up the bleeding. The steady hormones in the pill keep the uterine lining thin. When you skip the pill for a few days, the pill hormone levels go way down (it takes the body about 24 hrs to eliminate the hormones in one pill). This withdrawal of pill hormones destabilizes the uterine lining and you get spotting/bleeding [thus, the name: withdrawal bleeding].