Endometriosis is a condition that involves endometrial tissue developing outside a person’s uterus. The endometrium is the inner lining of the womb. Endometriosis can cause blood clots or fleshy tissue during periods.

This article goes into detail about endometriosis and blood clots, including symptoms, treatments, and causes.

Trigger warning: This feature mentions pregnancy loss, stillbirth, or both. Please read at your own discretion.

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Endometriosis is a condition that causes tissue usually in the uterus to grow on other organs and tissues. When a person has their period, this tissue sheds and bleeds in the same way endometrial tissue typically does.

However, the blood and tissue from endometriosis patches have nowhere to go. Instead, they remain inside the abdomen, causing inflammation and scarring.

When a person’s uterine lining sheds during their period, it can form small masses of blood and tissue. These are called menstrual clots. Usually, menstrual clots are smaller than a quarter.

Endometriosis can cause a person to have heavy menstrual bleeding. One of the symptoms of heavy bleeding is large menstrual clots.

However, heavy bleeding and menstrual clots can also be a symptom of a similar condition called adenomyosis. This is a condition that involves endometrial tissue growing in the muscle wall of the uterus.

Learn more

Learn more about endometriosis.

Menstrual clots can be in a person’s period blood. These clots can vary in size and color, and can be:

  • dark red
  • brown
  • jelly-like
  • stringy
  • thick
  • lumpy

Other endometriosis symptoms

In addition to menstrual clots, endometriosis can also cause:

The exact cause of endometriosis is unknown.

However, researchers believe that it may occur due to menstrual blood containing endometrial tissue flowing backward through the fallopian tubes.

From there, the blood travels to other areas of the body, depositing endometrial cells, which begin to develop.

Other possible causes of endometriosis include:

  • genetic factors
  • the immune system’s difficulty destroying endometrial tissue outside the womb
  • an estrogen hormone imbalance
  • abdominal surgery that results in the accidental deposit of endometrial tissue outside the womb

How does endometriosis cause menstrual clots?

Endometriosis can cause a person to produce additional estrogen. Estrogen is a hormone responsible for the regulation of a person’s periods. It also causes the uterus lining to become thick to prepare for pregnancy.

If a person has high estrogen levels, their endometrium becomes thicker. When a person has their period, this thick endometrium sheds, resulting in heavy blood flow and clots.

Other possible causes of menstrual clots

Menstrual clots can be a typical part of a person’s period. However, they can occur as a result of certain health conditions.

Other conditions that can cause heavy bleeding or menstrual clots include:

Certain factors may cause a person to be more likely to have heavy periods.

These include:

Adenomyosis is a similar condition to endometriosis. It involves the uterine lining cells growing inside the muscular wall of the uterus.

It can cause menstrual clots and other similar symptoms to endometriosis, including:

  • pelvic pain
  • heavy menstrual bleeding
  • painful period cramps
  • dyspareunia, or pain during sex
  • bleeding or spotting between periods

However, some people may not experience any symptoms. It is possible to have both endometriosis and adenomyosis.

Learn more about adenomyosis symptoms, diagnosis, treatment, and more.

The only way for a doctor to confirm that a person has endometriosis is by performing a laparoscopy.

A laparoscopy is a procedure that involves a surgeon making a small incision into a person’s abdominal skin.

The surgeon then inserts a device, called a laparoscope, into the cut. A laparoscope is a thin instrument that has a camera and a light on it. A surgeon uses the laparoscope to look for patches of endometriosis.

Before performing a laparoscopy, a healthcare professional will ask a person about their menstrual and medical history. The healthcare professional may then do a pelvic exam on the person and imaging tests.

Learn more

Learn more about diagnosing endometriosis.

Other menstrual problems

The National Institute of Child Health and Human Development notes that healthcare professionals can use the following methods to diagnose menstrual problems:

  • asking about medical history
  • physically examining the person
  • blood tests
  • ultrasound examination
  • endometrial biopsy
  • hysteroscopy, when the healthcare professional examines the inside of the person’s uterus
  • saline infusion sonohysterography, which involves filling the uterus with saline and imaging it with ultrasound
  • transvaginal ultrasonography, when a clinician inserts a device into the vagina to take ultrasound imaging

There is currently no cure for endometriosis. However, a person can manage their symptoms using various treatments.

Treatments for endometriosis include:

These methods can also treat heavy bleeding as a result of endometriosis. Additional treatments for heavy periods include:

  • tranexamic acid, a medication doctors use to stop excessive bleeding
  • relaxation techniques, such as meditation or yoga
  • surgery to remove the lining of the uterus or the uterus completely

The decidua develops when a person’s endometrium thickens to prepare for pregnancy.

If a person has a decidual cast, it means their decidua detached in one piece. The decidual cast then passes through the vagina, which can cause pelvic pain and cramping.

Decidual casts can result from an ectopic pregnancy. Occasionally, progesterone exposure — such as through oral contraceptives or hormone therapy — can cause a decidual cast.

Doctors often prescribe birth control pills that contain only progesterone to treat endometriosis. Some researchers believe that this increase in progesterone may increase the risk of passing a decidual cast. However, people must do more research to confirm this.

People with endometriosis also have a higher risk for ectopic pregnancy, which may increase their risk of passing a decidual cast.

There is currently no cure for endometriosis. However, treatment can help a person manage their symptoms. Condition information from 2022 found that endometriosis regressed by itself in a third of people with the condition.

However, endometriosis can result in a person having difficulties becoming pregnant. It can also cause a person to be at higher risk of ectopic pregnancy or pregnancy loss.

The same 2022 condition information above showed that endometriosis can reoccur in 6–67% of people who have surgery to remove it. Additionally, 5–59% of people continue to experience pain following treatment.

Support and information for people with endometriosis is available on the following sites:

It is advisable that a person contact a doctor if they have any symptoms of endometriosis. Additionally, a person may want to consider speaking with a doctor if they notice any unusual changes in their periods, such as

  • changes in color
  • an unusual odor
  • irregular periods
  • heavy bleeding
  • discolored discharge
  • severe pain or cramping
  • passing large clots

This section answers some frequently asked questions about endometriosis and menstrual clots.

Can endometriosis tissue come out during a period?

No, endometriosis tissue does not come out during a person’s period.

Instead, when the endometriosis tissue bleeds, it remains inside a person’s abdomen.

This can cause complications, such as inflammation and scarring.

What do periods look like with endometriosis?

If a person has endometriosis, their periods may be heavier and contain clots larger than a quarter.

Endometriosis is a condition that results in endometrial tissue growing outside the uterus. Having endometriosis can cause a person to have heavy periods. Heavy periods can result in large menstrual clots.

Endometriosis can cause a person to produce more estrogen than usual. This may cause thicker endometrial lining, leading to heavier periods and clots.

Certain health conditions can cause a person to have heavy menstrual bleeding. Hormonal birth control and obesity can also put a person at risk of heavy bleeding.

There is currently no cure for endometriosis. However, treatment can make the symptoms more manageable.

If a person has unusual changes in their periods, it is a good idea for them to contact a doctor.