Multiphasic combined oral contraceptive pills contain both estrogen and progestin, but the hormone dose changes during the active pills in the pack. Like other combined birth control pills, they are about 93% effective with typical use, meaning about 7 out of 100 people may become pregnant in a year, and about 99% effective with perfect use. They were designed to more closely mimic natural hormone changes during the menstrual cycle and may be chosen for people who have spotting, breakthrough bleeding, or side effects on other pills. They can be a little more confusing than monophasic pills because the order of the pills matters more. Different versions may vary by estrogen dose, progestin type, brand/generic version, bleeding pattern, cost, and side effects.