What Causes Endometriosis?

woman with endometriosis lying in bed with pain

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Endometriosis occurs when tissue similar to the type that lines the inside of the uterus (called the endometrium) grows outside of the uterus. The abnormal tissue growth is called "lesions" and these lesions can appear anywhere in your body, but commonly occur on your reproductive organs, colon, and bladder.

Currently, researchers don’t know why people develop endometriosis. However, experts have come up with several theories that describe why and how tissue begins to grow outside of the uterus.


As research on endometriosis remains ongoing, most theories currently point to a combination of genetic, environmental, and lifestyle factors that can increase your risk of developing the condition.

Retrograde Menstruation

Proposed by gynecologist John Sampson in 1927, retrograde menstruation used to be one of the more commonly accepted theories of endometriosis.

Menstrual bleeding happens when the endometrium leaves your body through the vagina. The theory of retrograde menstruation believed that menstrual blood could flow backward into the fallopian tubes, rather than exiting your vagina. From there, the menstrual blood and endometrial tissue would spread outside of your uterus.

However, this theory isn't as accepted anymore because endometriosis has been found in cisgender men and people assigned female at birth who have not yet begun menstruation—including fetuses.

Coelomic Metaplasia

Metaplasia occurs when one type of cell unexpectedly changes into another kind of cell. Experts who believe that coelomic metaplasia is an underlying cause of endometriosis suggest that cells outside of the uterus abnormally change into cells that look similar to endometrial cells. These cells then begin to grow and create lesions on nearby organs.

Some studies theorize that other factors like retrograde menstruation, hormonal imbalances, or a weak immune system also play a role in metaplasia, causing cells in other organs to create endometrial-like cells.

Vascular and Lymphatic Metastasis

In rare cases, endometrial-like cells can spread to the lungs and other organs that are further away from your reproductive system. To explain how endometriosis can spread to these body parts, Sampson also proposed the lymphatic and vascular metastasis theory. The vascular aspect of this theory states that capillaries (or, vessels that transport blood and oxygen to your organs) can carry endometrial cells to distant body parts. 

In some cases, endometrial-like cells can show up on the lymph nodes in your uterus. The lymph nodes are small bean-shaped structures located all throughout your body that are responsible for filtering out harmful substances that can travel in the fluid in your lymphatic system. The lymphatic aspect of this theory suggests that endometrial cells can travel to different parts of the body through your lymphatic fluid.

Stem Cells

Every cell in your body has a specific function that relates to the part of the body that creates those cells. However, stem cells have the ability to develop into several different types of cells. The function of stem cells is to repair, restore, and recreate any cells that your body may need to heal from illness or injury.

In your uterus, stem cells can help replace the endometrium that sheds through your vagina while you're on your period. According to this theory, stem cells can move outside of the uterus and develop endometrial-like cells which can cause lesions to develop and grow.

Embryonic Cells

During your prenatal (before birth) development, embryonic stem cells can develop into specialized cells. In most cases, these cells are placed in the Wolffian and Müllerian ducts—which are tubes that later become male and female reproductive organs, respectively. However, some embryonic cells might be left over, meaning they're not categorized to any part of your body yet.

These leftover or "residual" cells can continue to grow within and outside the vagina or uterus and turn into endometrial-like cells as your body produces estrogen. As a result, lesions may occur outside of your uterus, even before you experience your first period. Some experts believe that this theory can explain why some people develop endometriosis before menstruation.

Hormone Imbalances

Research has found that high levels of estrogen can worsen endometriosis symptoms. Aside from menstrual bleeding, those with endometriosis can also develop flares of painful symptoms, such as severe cramping, pelvic and abdominal pain, and changes to their menstrual cycle.

People with endometriosis who have higher estrogen levels or other hormonal imbalances may experience more pain and lesion growth than those with lower or suppressed levels of estrogen.


A handful of studies suggest that endometriosis may result from surgeries like a Cesarean section (C-section) or hysterectomy (the removal of the uterus). While more research is needed to support this theory, early studies theorize that a healthcare provider might accidentally move endometrial tissue outside of the uterus during surgery.

Is Endometriosis Hereditary? 

People with an immediate family member (e.g., mother or sister) with endometriosis are more likely to also develop the condition. By studying the genes of people with endometriosis, researchers have identified several genes that can affect your hormones and your risk of developing endometriosis.

One study published in 2016 in Human Reproduction found that the gene "vezatin" (VEZT) can raise your risk of receiving an endometriosis diagnosis. Usually, VEZT helps a fertilized egg cell attach inside the uterus at the start of pregnancy. However, researchers explain that some people with endometriosis have a mutation (change) in VEZT which can lead to the development of endometriosis.

Other genetic mutations can also increase your likelihood of developing endometriosis. These genes include, but are not limited to:

  • NPSR1, which can cause inflammation in the uterus
  • WNT4, which can lead to abnormal cell growth in the female reproductive organs
  • FN1/GREB1, which can change how much estrogen your body is producing

It's important to note that genetic testing is not yet available to assess your risk of endometriosis. However, as research continues, experts and healthcare providers both hope to find treatments that can reverse the effects of these genetic mutations.

Who Gets Endometriosis?

Anyone who menstruates can develop endometriosis. However, the condition is more common in people who:

  • Get their period before the age of 11
  • Have periods less than 27 days apart
  • Experience heavy periods or irregular menstruation that last longer than seven days
  • Are spotting in between their periods

Risk Factors

In addition to theories, researchers have identified several factors that may increase your risk of developing endometriosis. Your chances of experiencing symptoms may be higher if you align with one or more of the following risk factors.

  • Weight: Several studies have found a relationship between body mass index (BMI) and endometriosis risk. However, the exact relationship remains unclear. Some studies have found evidence that a low BMI can increase your endometriosis risk, while others believe that obesity may lead to the onset of symptoms. That said, more research is needed to understand the link between weight and endometriosis.
  • Age: People assigned female at birth between the ages of 15 and 40 are at an increased risk of developing endometriosis—mainly because this is a standard age range for reproduction.
  • Pregnancy: Some studies suggest that endometriosis can occur if you've never been pregnant. Typically, people who have never been pregnant have higher estrogen and lower progesterone levels than people who have been pregnant. As a result, having excess estrogen can lend evidence to the theory that a hormonal imbalance leads to endometriosis symptoms.
  • Lifestyle factors: While lifestyle factors alone can't cause endometriosis, it may be helpful to be aware of some lifestyle habits that may affect your biology and increase your risk of endometriosis. These lifestyle factors include getting less than three hours of exercise per week and drinking more than one alcoholic beverage a day.

Body Mass Index, or BMI, is a biased and outdated metric that uses your weight and height to make assumptions about body fat, and by extension, your health. This metric is flawed in many ways and does not factor in your body composition, ethnicity, sex, race, and age. Despite its flaws, the medical community still uses BMI because it’s an inexpensive and quick way to analyze health data.

A Quick Review

Endometriosis is a condition that occurs when the tissue that is similar to the type that lines the inside of the uterus starts to grow outside the uterus. As a result, you may experience a host of painful symptoms.

Why endometriosis occurs in the first place remains unclear. However, several theories suggest that a combination of genetic, environmental, and lifestyle risk factors can increase your risk of experiencing symptoms. These theories include retrograde menstruation, hormonal imbalances, and the way your stem cells function.

If you think you have symptoms of endometriosis or may be at a higher risk of developing the condition, it's a good idea to visit your healthcare provider and talk about your symptoms. Your provider can get you started on testing, which may help you get a diagnosis sooner and support you in getting the treatment that you need to better manage your symptoms.

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13 Sources
Health.com uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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