Why do I menstruate while on birth control?

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, such as Tri-cyclen, use synthetic hormones to prevent ovulation and thicken the cervical mucus — both of which help keep sperm from fertilizing an egg, thus preventing 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. This is called withdrawal bleeding. 

So, here's how it works: normally, a 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, thus the person from becoming pregnant.

Furthermore, you asked why do people still menstruate while taking the pill. The answer to this question has to do with the design of birth control pills. When the pill was manufactured in 1958, it was designed to mimic a typical 28-day menstrual cycle, which is primarily the reason why birth control packets contain a combination of hormonal pills and placebo 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 thickened.

According to many reproductive health experts, menstruation serves no biological purpose if a person is on birth control. In fact, a person can purposefully skip their period by omitting the placebo week and starting a new pack of pills, patch, or ring. Birth control manufacturers have caught onto some people'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. You can check out Can I reschedule my period for more information about other birth control options that can help regulate periods.

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

Last updated Feb 18, 2022
Originally published Feb 06, 2009

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