Many people will use birth control pills at some time in their lives. In addition to preventing pregnancy, there are many other benefits and medical uses for birth control pills. Birth control pills are made of synthetic (laboratorymade) versions of two hormones produced by the ovary: progesterone and estradiol. Some pills contain both of these hormones (combined oral contraceptives) whereas others contain only a synthetic version of progesterone, called a progestin (progestin-only pills). There are a variety of reasons why a provider may choose one type of birth control pill over another. In general, progestin-only pills are great for people who do not want to, or cannot, take estrogen; however, they may cause unpredictable vaginal bleeding rather than a predictable monthly period.

To understand how birth control pills affect periods, it is helpful to understand how the normal menstrual cycle works.