The placebo, or hormone-free, week in the 28-day pill package simulates an average menstrual cycle, though the hormonal events during a pill cycle are significantly different from those of a normal ovulatory menstrual cycle. Because the pill suppresses ovulation (to be discussed more in the Mechanism of Action section), birth control users do not have true menstrual periods. Instead, it is the lack of hormones for a week that causes a withdrawal bleed. The withdrawal bleeding that occurs during the break from active pills has been thought to be reassuring, a physical confirmation of not being pregnant. The withdrawal bleeding is also predictable. Unexpected breakthrough bleeding can be a possible side effect of longer term active regimens.