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Jan 2023

Why do I bleed monthly?

Whether you are frustrated by the fact that menstruators have to “deal” with periods, or you’re just curious about your body, in this issue of Periodically we will take a dive into why humans menstruate cyclically. Let’s start off by understanding what menstruation is and what happens during this important time of the month. See the table below to learn about different theories that circulate in scientific communities to answer this question. And, then read about some of the evolutionary benefits of a monthly period.

Leading Theories About The Menstrual Cycle

Read about the four main theories that scientists have as to why menstruators bleed every month. It's important to understand that these are still working hypotheses that scientists continue to research about.

Shedding the uterine lining helps remove germs that may have entered from sexual intercourse

While a period can remove some germs, periods aren't based on sexual activity. Also, not all animals menstruate cyclically (like we do) but most still have sperm




Sperm-borne pathogen removal

Non-adaptive consequence of

spontaneous decidualization

This theory would still not explain the evolutionary benefit of menstruation.

Menstruation is simply a consequence (or side effect) of the uterus building up the lining in preparation for a new pregnancy

Energy conservation

Menstruation is less energetically costly than continuously maintaining a lining until an egg is fertilized

No species continuously keeps up a lining, even ones that don’t menstruate, so that’s not an alternative in the first place. This also would not allow for sperm to reach the egg

Uterine preconditioning

Menstruation prepares the uterine tissues for the inflammation and oxidative stress that comes with pregnancy.

There isn’t much, if any, evidence to support this theory or that menstruating protects against the stress of pregnancy.

Why Do I Bleed Every Month?

Editor: Anika Bukkapatnam 
Writer: Annmarie Calderon, MPH
Publisher: spotBOX LLC. 

Every 28 days, on average, the uterus builds up a lining of tissue on its inner surface, the endometrium, to prepare for a potential pregnancy.

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the uterus, located behind the bladder and rectum, builds an endometrial lining. This endometrium, serves as a ‘soft landing pad,’ for a fertilized egg (Hill).

   Every month that a viable fertilized egg is not implanted, the body gets rid of the egg and uterine lining through the process of menstruation (What Is Menstruation?, n.d.). Scientists still do not have a definitive answer as to why we menstruate every thirty-ish days. However, there are a few theories to answer this question, with some being more probable than others (Emera et al.).

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uterine lining/endometrium

Endometrial Lining developing

   Under the evolutionary microscope, menstruation is very costly to our body’s resources (think about the materials it needs to build up the lining, only to tear it down and get rid of it a few weeks later). So, there must be a good reason for why menstruating bodies have evolved to shed their endometrial lining every month. One theory that stands out actually takes us back to the primary goal of all organisms: passing down our genes!

   While pregnant, mothers devote a significant portion of their body’s resources to their child. However, these resources come at a loss if the fertilized egg is inviable.

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To prevent unnecessary energy expenditure, the uterus puts the fertilized egg through a survival test- the fittest eggs will survive.

   A viable implanted embryo that passes the 'survival test' will then release enough human chorionic gonadotropin (or hCG) to prevent the body from shedding it (Betz and Fane).   

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human chorionic gonadotropin

luteinizing hormone


(metabolic inhibitor of the hormone, progesterone)

   Regardless of which theory is truly accurate, it remains true that menstruation is much less energetically costly than sustaining a pregnancy. Nevertheless, scientists continue to explore the question of why humans have evolved to menstruate as they do. It is paramount to stay on top of the newest research for the benefit of one's own health and wellbeing.

Enjoyed what you read? Check out these resources!

"Human Chorionic Gonadotropin"

"Characteristics of the Endometrium in Menstruating Species: Lessons Learned from the Animal Kingdom"

"The Evolution of Menstruation: A New Model for Genetic Assimilation"

"This Is Your Brain on Birth Control : The Surprising Science of Women, Hormones, and the Law of Unintended Consequences" by Sarah E. Hill

"What Is Menstruation? | Get Facts about Having Your Period"


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What are the most effective forms of contraception?

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Why do I get a menstrual period every month?

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