Birth control pills, commonly called "the pill" are used to prevent pregnancy and regulate a woman's menstrual cycle. Typically, a woman will take one active (hormone-containing) pill per day for three weeks, then will take either no pills or sugar pills for one week. Menstrual bleeding usually occurs during the week that no hormones are taken. Occasionally, a woman on birth control pills will experience a missed period, called amenorrhea. There are several reasons why you might experience a missed period while on birth control pills.
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According to Brown University, the hormones in the birth control pills may suppress your period for one or more months. When this happens, see your doctor, as a different birth control pill may allow you to menstruate regularly. There are also birth control pills that are supposed to stop your menstrual period for a period of time. Seasonale, for example, only causes a period every three months and Lybrel can stop your periods until you stop taking it.
The University of Michigan's Health Center lists certain illnesses that may cause you to miss a period. These include tumors, false pregnancy, thyroid diseases, ovarian cysts, depression and chronic illnesses such as kidney disease or cystic fibrosis. It recommends seeing your health care professional if you miss two periods in a row.
When you miss your period while on birth control pills, be sure to consider any other medications that you are taking which might interfere with your menstrual cycle. According to the Mayo Clinic, medicines such as oral steroids, chemotherapy, antidepressants and antipsychotics may all cause amenorrhea. Ask your doctor whether these medications may cause you to miss your period if you are taking other medication besides birth control pills.
Sometimes, birth control pills fail, and you may become pregnant. According to the Feminist Women's Health Center, this is more likely if you have been on an antibiotic, if you missed pills or started late, or if it was your first month on birth control pills. According to Cornell Health Services, having a stomach bug that causes diarrhea or vomiting (and therefore prevents the proper absorption of the hormones in the pill) may also cause you to become pregnant. Symptoms of pregnancy besides a missed period include sore breasts, nausea and frequent urination. Take a home pregnancy test or see your doctor before starting your next pack of pills if you think that you may be pregnant.