November 15, 2019

What Causes the Uterine Lining to Shed?

Why exactly does this happen every 23-30 days? Let’s discuss.
Written by
Dr. Sarah Toler
Published on
November 15, 2019
Updated on
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Most people with uteruses shed their uterine lining every 23 to 30 days. This is what most people refer to as “getting your period” and is the final step in the menstrual cycle.

During menstruation, the reproductive organs are regulated by hormones. The sex hormones estrogen, progesterone, and androgen work together to keep your menstrual cycle repeating every month or so. Reproductive hormones are synthesized in a complex cycle involving two parts of the brain called the hypothalamus and the anterior pituitary, as well as the ovaries. 

The functions of the ovaries are one half of the menstrual cycle. The other half of the menstrual cycle takes place in the uterus and concludes with the shedding of the uterine lining.

What causes the uterine lining to shed? 

The uterine lining, also called the endometrium, is the layer of tissue lining the inside of your uterus. It consists of epithelial cells, the same types of cells that make up your skin and cover your entire body. They protect your internal organs from the world outside. 

The Proliferative Phase and the Secretory Phase

The endothelial cells making up your endometrium react to the hormonal fluctuations of the menstrual cycle. The first half of your cycle is called the proliferative phase. During this phase, the cells of the endometrium respond to increases in estrogen by lengthening and linking to one another. During ovulation, the secretory phase begins. Progesterone increases blood flow to the endometrium and the tissue swells. 

The Ischemic Period

By day 25 of the average cycle (if pregnancy hasn’t occurred), estrogen and progesterone both decrease. This time is called the ischemic period — ischemia is the word used when tissue doesn’t receive enough blood supply. The endometrial tissue shrinks and blood flow is restricted. This causes the uterus to contract to shed this tissue that’s no longer receiving adequate oxygen. 

The lining of the uterus then begins to shed. This is the time when a person will first notice the appearance of menstrual blood. Bleeding will occur for several days until the entire lining has been replaced with new cells, ready for another cycle. 

What happens if you don't shed your uterine lining 

If you’ve had a period before and suddenly stop menstruating, a number of things might be happening. Pregnancy is usually the first thing that should be ruled out. Other causes include breastfeeding, some forms of birth control and menopause.

When these causes have been ruled out, you might not be having a period because you’re not ovulating. Lack of ovulation can be caused by a condition called polycystic ovary syndrome  (PCOS). If you haven’t had a period in over three months, visit your doctor and they can run some tests for PCOS. 

High levels of stress and changes in weight, diet or exercise habits can cause menstrual cycle disruptions. Eating disorders and illnesses can also cause changes to your period. This occurs because any of these changes can impact the hypothalamus and result in what’s called functional hypothalamic amenorrhea.

Some forms of birth control like intrauterine devices (IUDs), the shot and the progesterone implant can cause you to skip periods. Researchers generally agree that it’s not harmful to skip periods while on birth control. 

What we know about the uterine lining

So, how do we know what the inside of a uterus looks like? Researchers have studied endometriums in a few ways. One way is to take a biopsy of the uterus, which is the process of removing a tiny bit of tissue and studying it under a microscope. Doctors can now use a specialized camera called a hysteroscope to study the ways in which hormones affect this tissue. 

Menstruation and hormones have only been studied for a short time. While our knowledge of the uterine lining and the menstrual cycle has grown exponentially, we still have a lot to learn. Researchers have yet to come to a conclusion about the necessity of menstruation. Some think it rids the uterus of bacteria, while others think there’s no purpose to periods at all. 

Is it bad to shed the lining of your uterus?

Absolutely not! Science can’t tell us everything about hormones, but we know hormones cause changes to the tissue in the uterus that leads to your period. The lining of your uterus sheds in response to hormones that reduce the blood flow to the endometrial tissue. Most people with uteruses shed their uterine lining roughly every 28 days. 

It seems natural to be a little concerned about shedding a body part, but that’s the way the human body was designed to function. Did you know your skin also renews itself every 2 to 4 weeks? The human body has some pretty impressive ways of regulating itself. Your period and shedding the lining of your uterus is just one of these functions.

Reviewed for Medical Accuracy

Dr. Toler specializes in reproductive mental health and focuses on the intersections where reproductive rights, mental health, healthcare access and equality meet. Through creating evidence-based health content, she hopes to improve access to reproductive health information for all people. She is a perinatal mental health advocate and currently contributes to several perinatal mental health action groups including Maternal Mental Health NOW and Postpartum Support International.

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