Touted as one of the greatest human inventions in history, birth control pills (also known as the pills or oral contraceptives) are now taken by 12 million women in America. The pills, essentially female hormones, prevent pregnancy by keeping the eggs from being released by the ovary. They also keep the sperm at bay by thickening women’s cervical mucus. Birth control pills are very effective with a 99% success rate if taken every day as directed.

The pills are classified by how many hormones they contain; combination pills have two (estrogen and progestin), while others have just one (progestin).

Combination pills come in a 28-day or 21-day pack. Both have 21 “active” pills, and the last 7 pills in the 28-day pack are called “reminders”, dummy pills just to help maintain the habit. In a 21-day pack, one pill is taken every day for 3 weeks, no pills for the next week, and then a new pack is started. You get periods during the 4 ^th week. There are also newer combination pills (Yaz 28 and Loestrin 24 Fe) that have 24 days of active pills, followed by 4 days of dummies.

Some combination pills contain a few months’ supply of active pills. They are specially packaged to reduce the number of periods over the year. The first extended-cycle pill regimens provide active pills every day for 3 months, then 1 week of dummy pills (Seasonale) or low-dose estrogen pills (Seasonique, LoSeasonique). Menstrual bleeding occurs during that week. A newer extended-cycle regimen is designed to be taken continuously for 1 year (Lybrel). It’s meant to suppress all menstrual bleeding.

Progestin-only pills come only in 28-day packs. All the pills are active, so it is critical that you take these pills same time every day. You may get a period the 4 ^th week, or no periods, or have bleeding on and off throughout the month.