Why do I menstruate while on birth control?

Originally Published: February 6, 2009 - Last Updated / Reviewed On: March 26, 2013
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Dear Alice,

I read on the Tri-cyclen website that the hormones mimic pregnancy and trick the body into thinking that I'm pregnant so that an ovum isn't released for fertilization, or in other words, no ovulation. If that's the case, why do I still menstruate? I thought that the only reason why I menstruate is to expel the unfertilized egg. Can you clarify?

—Curious

Dear Curious,

You're on the right track. Birth control pills like Tri-cyclen use synthetic hormones to prevent ovulation and thicken the cervical mucus — both of which help keep sperm away from an egg and thus prevent pregnancy. Technically speaking, there's no need for menstruation if you're on the pill. However, most birth control packs contain placebo pills that cause monthly bleeding similar to a period. Here's how it works:

Normally, a woman's menstrual cycle is regulated by the ebb and flow of several hormones. Each month, these hormones signal the uterus to grow an extra cushy lining to welcome a fertilized egg. If fertilization doesn't occur, then the uterine lining, or endometrium, is shed as menstrual fluid. In a way, your period is the body's way of cleaning house to get ready for the next possible pregnancy. The hormones in birth control pills prevent ovulation and also stop the uterine lining from growing. So you ask, why do women still menstruate while taking the pill?

The answer has to do with the history of birth control. In 1958, two doctors named John Rock and Gregory Pincus revolutionized contraception with the first clinical trials of oral contraceptives. Rock and Pincus decided that the pill would be more acceptable to women (and organizations like the Catholic Church) if it preserved women's natural menstrual cycle. So, they manufactured the pill to mimic a typical 28-day cycle. This is why many birth control packets contain three weeks worth of hormonal pills and one week of placebos or sugar pills. Withdrawal from the hormones on the fourth week triggers bleeding that's similar to menstruation. However this "withdrawal bleeding" is usually shorter and lighter than a regular period because the uterine lining hasn't been thickened.

According to many women's health experts, menstruation serves no biological purpose if a woman is on birth control. In fact, a woman can purposefully skip her period by omitting the placebo week and starting a new pack of pills, patch, or ring. Birth control manufacturers have caught onto some women's desire to have less frequent periods, and there are now several brands of the birth control pills on the market that don't have a placebo week. Check out Can I reschedule my period for more information.

Hopefully this info fed your curiosity and cleared up any confusion about the pill!

Alice