Understanding How Common Endometriosis Really Is
Endometriosis is far more widespread than many people realize. This condition affects an estimated 10% of women of reproductive age across the globe, which translates to approximately 190 million women worldwide[2]. In the United States alone, between 5 million and 10 million people are living with this disease[11]. The condition is particularly common among women in their 20s and 30s, though healthcare providers can diagnose it at any point from the first menstrual period through menopause[3].
When looking at specific groups, the numbers become even more striking. Among people who do not have obvious symptoms, the prevalence ranges from 2% to 11%. However, when examining individuals who struggle with infertility, the rate jumps dramatically to between 5% and 50%[11]. The picture is especially concerning for young women experiencing severe menstrual pain. Among teenage girls with chronic pelvic pain, studies estimate that 50% to 70% may have endometriosis, and for those whose pain does not respond to standard medical treatment, the rate climbs to as high as 75%[11].
It is important to note that these statistics likely underestimate the true scope of the problem. Because definitive diagnosis requires surgical visualization, many people with endometriosis may never pursue or receive a formal diagnosis[11]. The condition affects women regardless of ethnic origin or social status, and can also impact transgender men and non-binary individuals who menstruate[2].
What Causes Endometriosis
Despite years of research, healthcare providers still do not know for certain what causes endometriosis. The condition occurs when tissue that is similar to the endometrium (the lining of the uterus) grows in places outside the uterus where it does not belong[1]. This misplaced tissue responds to hormonal changes during the menstrual cycle, just like the tissue inside the uterus does. However, when this tissue breaks down and bleeds, it cannot easily leave the body, leading to inflammation and the formation of scar tissue[2].
Several theories attempt to explain why endometriosis develops. The most widely accepted explanation is known as Sampson’s theory or the theory of retrograde menstruation. This theory suggests that during menstruation, some menstrual blood flows backward through the fallopian tubes into the peritoneal cavity instead of leaving the body through the vagina. The viable cells in this fluid can then implant, grow, and infiltrate the peritoneal cavity (the space inside the abdomen that contains organs like the intestines and liver)[5]. Retrograde menstruation is actually quite common among women of reproductive age, though not all women who experience it develop endometriosis[5].
Another explanation is the coelomic metaplastic theory, suggested by Meyer. This proposes that certain cells in the body have the ability to transform into endometrial-like tissue[5]. Other theories include the possibility that endometrial cells travel through blood vessels or the lymphatic system to reach distant parts of the body, or that stem cells are involved in the development of the condition[5].
Emerging research points to problems with the immune system as a contributing factor. Studies show that endometriosis is associated with immune system dysregulation (when the immune system does not function properly). People with endometriosis have higher rates of other immune-related conditions, such as lupus, multiple sclerosis, and inflammatory bowel disease[2]. Current thinking suggests the disease involves complex interactions between various systems in the body, including hormones, the immune system, inflammatory processes, and factors that promote the growth of new blood vessels[11].
Risk Factors That Increase Your Chances
While the exact cause of endometriosis remains unclear, researchers have identified several factors that can increase the likelihood of developing the condition. One of the strongest risk factors is family history. If a close biological relative such as a mother, grandmother, or sister has endometriosis, your risk of developing the condition increases significantly[3][9]. This genetic connection suggests that certain inherited factors may make some women more susceptible to the disease.
Certain characteristics of the menstrual cycle are also associated with higher risk. Women who have never given birth appear to be at increased risk. Additionally, those whose menstrual cycles occur more frequently than every 28 days, or who have heavy and prolonged periods lasting longer than seven days, face a higher likelihood of developing endometriosis[1]. Having higher levels of estrogen in the body is another risk factor, which makes sense given that endometriosis is considered an estrogen-dependent condition[1].
The age at which menstrual periods begin can also play a role. Starting menstruation at an early age may increase risk, though the relationship between age and endometriosis is complex. While the condition most commonly affects women in their reproductive years, particularly those in their 20s and 30s, it can develop at any age from puberty onward[3][9]. Even though many women find relief from symptoms after menopause, endometriosis can still cause discomfort and pain in postmenopausal women[3][9].
Recognizing the Symptoms
The symptoms of endometriosis can vary widely from person to person, and their severity does not always match the extent of the disease. Some women have extensive endometriosis but experience little to no pain, while others have minimal tissue growth but suffer severe symptoms[3][9]. This unpredictability makes the condition challenging to identify and understand.
The most common symptom is pelvic pain. This pain is often severe and typically worsens just before and during menstruation due to inflammation caused by hormonal changes[3]. Many women describe extremely painful menstrual cramps that can stop them from carrying out normal daily activities[2]. The pain may be felt in the lower abdomen, back, or even radiate into the legs[7]. Some women characterize it as cramp-like pain, which might be accompanied by nausea, vomiting, and diarrhea[7].
Heavy menstrual bleeding is another hallmark symptom. Women with endometriosis may need to change pads or tampons every one to two hours, or may bleed through to their clothes[4]. Some also experience spotting or light bleeding between periods[3]. This excessive blood loss can lead to anemia, which causes extreme tiredness, dizziness, and light-headedness[21].
Chronic pelvic pain that persists even when menstruation has ended is common. This ongoing discomfort can significantly affect quality of life[2]. Pain during or after sexual intercourse, medically known as dyspareunia, is frequently reported and can strain intimate relationships[3][6].
When endometrial tissue grows on or near the bladder or bowel, it can cause pain during urination or bowel movements[3][4]. Women may also experience digestive symptoms such as diarrhea, constipation, bloating, and nausea, particularly during their periods[2][3]. These symptoms can sometimes lead doctors to initially suspect digestive disorders rather than endometriosis.
Infertility is a serious concern for many women with endometriosis. The condition can make it difficult to become pregnant due to scarring and blockage of the fallopian tubes[3][9]. In fact, some women only discover they have endometriosis when they struggle to conceive and seek medical evaluation[3].
Beyond physical symptoms, endometriosis often takes a toll on mental health. Many women experience low mood, anxiety, and depression related to chronic pain, uncertainty about the condition, and concerns about fertility[2][4]. Extreme fatigue is also commonly reported, which can further impact daily functioning and emotional well-being[4].
In rare cases, endometriosis affects organs outside the pelvis. When tissue grows in the chest area, such as in the lungs or diaphragm, it can cause shortness of breath and coughing up blood[4][7]. It is worth noting that some women with endometriosis remain completely asymptomatic and may only be diagnosed during surgery for another reason or during infertility investigations[2].
Prevention Strategies
Unfortunately, because the exact causes of endometriosis are not fully understood, there is no guaranteed way to prevent the condition from developing. However, understanding the risk factors can help women and their healthcare providers remain vigilant for early signs of the disease.
Awareness of family history is crucial. Women who have biological relatives with endometriosis should inform their healthcare providers, as this knowledge can lead to earlier investigation of symptoms and potentially quicker diagnosis[3][9]. Early detection and treatment, while not preventing the condition itself, can help prevent the progression of symptoms and limit long-term complications such as severe scarring or infertility.
Some research has explored whether factors that lower estrogen exposure might reduce risk. For example, regular exercise and maintaining a healthy weight may have protective effects, though the evidence is not conclusive. Similarly, some studies suggest that long-term use of hormonal contraceptives might reduce the risk of developing endometriosis or delay its onset, but more research is needed in this area.
The most important preventive measure is awareness and early medical attention for symptoms. Women should not dismiss severe menstrual pain as normal. Seeking help promptly when experiencing symptoms such as debilitating period pain, heavy bleeding, or chronic pelvic discomfort can lead to earlier diagnosis and intervention, potentially preventing the condition from worsening and reducing the impact on fertility and quality of life[2].
What Happens in the Body
Understanding the pathophysiology of endometriosis means looking at how the disease changes normal body functions. In a healthy woman, the endometrium builds up inside the uterus each month during the menstrual cycle. If pregnancy does not occur, this tissue breaks down and leaves the body as menstrual blood through the vagina[7].
In endometriosis, tissue that looks and behaves like the endometrium grows in locations outside the uterus. The most common sites include the ovaries, fallopian tubes, the outer surface of the uterus, the space behind the uterus, and the peritoneum (the membrane lining the abdominal cavity)[1][3]. Less commonly, endometrial-like tissue can be found in the rectum, bladder, intestines, diaphragm, vagina, and even the lungs[3].
This wayward tissue responds to the same hormones that control the menstrual cycle, particularly estrogen and progesterone. During menstruation, the misplaced tissue breaks down and bleeds just as the uterine lining does. However, unlike menstrual blood from inside the uterus, this blood has nowhere to go. It becomes trapped, pooling around nearby organs and tissues[1][7].
The trapped blood and tissue cause irritation and inflammation in the surrounding areas. Over time, this chronic inflammation leads to the formation of scar tissue and sticky fibers called adhesions that can bind tissues and organs together[1]. These adhesions can cause organs that are normally separate to stick to each other, which can be painful and affect organ function.
In some cases, endometriosis causes the formation of cysts on the ovaries, known as endometriomas or “chocolate cysts” because they contain old, dark blood that resembles chocolate[1][5]. These cysts can grow large enough to damage healthy ovarian tissue.
The pain associated with endometriosis comes from multiple sources. The inflammation itself causes discomfort. Additionally, researchers have observed that endometriotic tissue contains high levels of nerve growth factors, which lead to the development of new sensory and sympathetic nerve fibers in and around the lesions. This process, called neurogenesis, means that the affected areas become more sensitive to pain[11]. In some cases, nerve fibers become entrapped within endometriotic lesions, which may explain why some women experience pain that radiates to areas like the lower back and legs.
The immune system also plays a significant role. In women with endometriosis, the immune response appears to be altered. Rather than recognizing and removing the misplaced endometrial tissue, the immune system may contribute to the inflammatory environment that allows the tissue to survive and grow[2]. This immune dysfunction helps explain why endometriosis is a chronic, progressive disease.
Estrogen is particularly important in the development and maintenance of endometriosis. The hormone promotes the growth of endometrial tissue through effects that encourage cell proliferation and prevent cell death. Estrogen also stimulates both local inflammation in the endometriotic tissue itself and systemic inflammation throughout the body[11]. This is why many treatments for endometriosis focus on reducing estrogen levels or blocking its effects.
The impact on fertility occurs through several mechanisms. Adhesions can distort the normal anatomy of the reproductive organs, making it difficult for an egg to travel from the ovary through the fallopian tube to the uterus. Inflammation may create an environment that is hostile to sperm or embryos. In severe cases, endometriosis can damage the ovaries themselves, reducing the number or quality of eggs available for fertilization[7].
